==== Front Front Psychiatry Front Psychiatry Front. Psychiatry Frontiers in Psychiatry 1664-0640 Frontiers Media S.A. 10.3389/fpsyt.2023.1109336 Psychiatry Review Key insights from studies on the stability of personality disorders in different age groups d’Huart Delfine 1 * Seker Süheyla 1 Bürgin David 1 2 Birkhölzer Marc 3 Boonmann Cyril 1 3 4 Schmid Marc 1 Schmeck Klaus 1 Bach Bo 5 1Department of Child and Adolescent Psychiatric Research, Psychiatric University Hospitals Basel, Basel, Switzerland 2Department of Child and Adolescent Psychiatry and Psychotherapy, Ulm University, Ulm, Germany 3Department of Forensic Child and Adolescent Psychiatry, University Psychiatric Clinics Basel, Basel, Switzerland 4LUMC Curium—Department of Child and Adolescent Psychiatry, Leiden University Medical Center, Leiden, Netherlands 5Center of Excellence on Personality Disorder, Psychiatric Research Unit, Region Zealand, Slagelse Psychiatric Hospital, Slagelse, Denmark Edited by: Lionel Cailhol, University Institute in Mental Health of Montreal, Canada Reviewed by: Felix-Antoine Berube, Montreal University, Canada; Yann Le Corff, Universitéde Sherbrooke, Canada *Correspondence: Delfine d’Huart, Delfine.d’Huart@upk.ch 15 6 2023 2023 14 110933627 11 2022 25 5 2023 Copyright © 2023 d’Huart, Seker, Bürgin, Birkhölzer, Boonmann, Schmid, Schmeck and Bach. 2023 d’Huart, Seker, Bürgin, Birkhölzer, Boonmann, Schmid, Schmeck and Bach https://creativecommons.org/licenses/by/4.0/ This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. While for decades, temporal stability has been conceived as a defining feature of personality disorders (PDs), cumulative findings appear to question the stability of PDs and PD symptoms over time. However, stability itself is a complex notion and findings are highly heterogenous. Building upon a literature search from a systematic review and meta-analysis, this narrative review aims to capture key findings in order to provide critical implications, both for clinical practice and future research. Taken together, this narrative review revealed that unlike previous assumptions, stability estimates in adolescence are comparable to stability estimates in adulthood and PDs and PD symptoms are not that stable. The extent of stability itself depends yet on various conceptual, methodological, environmental, and genetic factors. While findings were thus highly heterogenous, they all seem to converge in a notable trend towards symptomatic remission, except for high-risk-samples. This challenges the current understanding of PDs in terms of disorders and symptoms and argues instead in favor of the AMPD and ICD-11 reintroducing the idea of self and interpersonal functioning as the core feature of PDs. personality disorders personality disorders symptoms mean-level stability rank-order stability review Fonds National de la Recherche du Luxembourg (FNR) section-at-acceptancePersonality Disorders ==== Body pmc1. Introduction Traditionally conceived as a defining feature of personality disorders (PDs), stability has quickly become a major concern, adding to the ongoing debate about the procedure of conceptualizing and diagnosing a PD. For decades, temporal stability has been a major factor in distinguishing axis I from axis II disorders with the stability of PDs being considered to be higher than for other mental disorders. Cumulative findings, however, gradually challenged the stability of PDs, indicating a notable trend towards improvement over time (1, 2). Unlike previous assumptions, PDs have thus not been found to be much more stable than other mental disorders (3). Nevertheless, stability is a complex notion that should be assessed in the light of several factors (4, 5). As such, PDs may be conceptualized in multiple ways including categories, symptom counts, and pathological traits. Similarly, various conceptually and statistically distinct approaches may lead to distinct types of stability. These different types, then, may depend on various methodological factors, such as sampling procedures (i.e., age range, clinical status, follow-up interval), the assessment modality, and the type of instrument being used. As a result, study findings are highly heterogenous, and misconceptions about the course of PDs still seem to remain. In this narrative review, we capture key findings of the current literature on the stability of PDs across different age groups and critically discuss general implications for both clinical practice and future research. We start by describing different PD constructs and different types of stability, followed by an overview of recent studies in childhood, adolescence, and adulthood. Finally, we emphasize key findings and conclude with general implications. 2. Personality disorder constructs PDs can be conceptualized according to different constructs, features, and frameworks. As such, in both the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders [DSM-5 (6)]; and the 10th edition of the International Classification of Disorders [ICD-10 (7)], PDs are defined as discrete categories, each with a distinct set of diagnostic criteria (i.e., either a PD is present or not). Within this categorical system, PDs can also be conceptualized more dimensionally, in terms of symptom counts (e.g., seven out of nine borderline PD symptoms). In recent PD models, such as the Alternative Model of Personality Disorders (AMPD) in section III of the DSM-5 (6), as well as the 11th edition of the ICD (7), PDs are, moreover, perceived in terms of core impairments in personality functioning (i.e., self-, and interpersonal functioning), specified by a set of pathological traits (i.e., extreme variants of normal personality dimensions, such as emotional lability, attention seeking or impulsivity). These different constructs and approaches may naturally affect stability estimates. Although a growing number of longitudinal studies investigate dimensional measures of personality pathology [e.g. (8, 9)], previous research has focused primarily on PD categories and PD symptoms counts, except for child-and adolescent studies focusing exclusively on maladaptive personality traits. Therefore, the current review focusses exclusively on DSM and ICD based categorical and symptom-based models. 3. Different types of stability Apart from the aforementioned constructs, multiple ways to describe stability over time are common, and stability itself tends to differ according to the type of stability assessed. In the present review, we focus on the two types of stability that have been studied most frequently, namely, mean-level and rank-order stability. Mean-level stability refers to the degree to which the average level of a PD or PD symptom changes over time. Categorical mean-level stability, also known as diagnostic stability, then refers to the consistency of PD diagnoses, typically measured through the proportion of enduring cases from baseline to follow-up (e.g., four out of ten participants, who were diagnosed with BPD at baseline, still meet the criteria at follow-up, resulting in a categorical mean-level stability of 40%). Dimensional mean-level stability then refers to the consistency of PD symptom counts, usually measured by mean-difference scores (i.e., difference between mean symptom count at follow-up and mean symptom count at baseline). Rank-order stability, in turn, refers to the consistency of an individual’s relative ordering compared to others in a given sample, indicating thus the degree to which interindividual differences are preserved over time. As such, individuals may retain their relative ordering with regard to a specific PD or PD symptom over time, even if the average level of a PD or PD symptom in a given sample increases or decreases over time. Subsequently, rank-order changes are independent of mean-level changes (10). Categorical rank-order stability then refers to the rank-order stability of individuals’ PD diagnosis, typically measured with Cohen’s κ. While a negative value indicates no agreement, a κ between 0 and 0.20 indicates a low, a κ between 0.21 and 0.40 a fair, and a κ between 0.41 and 0.60 a moderate agreement. A κ between 0.61 and 0.80, then, indicates a substantial agreement, and a κ between 0.81 and 1.0 a perfect agreement (11). Dimensional rank-order stability, in turn, refers to the rank-order stability of an individuals’ PD symptom count, commonly measured through a test-retest correlation (e.g., Pearson’s r). A r between 0.1 and 0.3 is said to be low, a r between 0.3 and 0.5 moderate, and a r between 0.5 and 0.8 high (12). Another powerful method to assess the stability of PDs over time, consists in using structural equation models. Structural equation models encompass a set of multivariate approaches [e.g., individual growth curve models (13); growth mixture modeling (14)] that allow to distinguish between measurement error and true individual differences related to change processes. 4. Overview of the current literature review The literature search for this narrative review was part of a systematic review and meta-analysis, conducted in accordance with the PRISMA standards (15) as well as the MOOSE guidelines (16). The literature search conducted in four electronic databases (EMBASE, PsycInfo, PubMed, and Web of Science) on October 26, 2020, and updated on June 7, 2022 (d’Huart et al., under review). Keywords and Medical Subject Headings (MeSH) terms were used to identify peer-reviewed articles reporting on the stability of PDs between 1980 and 2022. In brief, following search terms were used in the literature search: “personality disorders,” “axis II disorders,” “stability,” “consistency,” “longitudinal,” “prospective,” “life span,” and “life course.” Only longitudinal studies, assessing the stability of PDs at two different time points at least 1 month apart, were considered for the current paper. Studies will be presented from a developmental perspective, including childhood, adolescence, and adulthood. A complete overview is given in Tables 1–3. Table 1 Longitudinal studies on the course of PDs in childhood (k = 2). Author(s) and publication year Sample sizea Time intervalc Mean ageb M (SD) Setting Assessment of PDs and PD traits Main outcome PD construct Type of stability Type of PD Instrument Crick et al., 2005 (17) 400 24 NR Clinical Traits Rank-order (D) BPD BPFS-C Moderate dimensional rank-order stability de Clercq et al., 2009 (18) 477 12 10.67 Clinical Traits Mean-level; Rank-order (D) BPD DIPSI The children’s maladaptive trait scores generally decreased as they grow older; substantial dimensional rank-order stability BPD, borderline PD; BPFS-C, borderline personality features scale for children; DIPSI, dimensional personality symptom item pool; NR, not reported. a Sample size used for the analyses. b Mean age at baseline. c The follow-up interval is displayed in months. Table 2 Longitudinal studies of the course of PDs in adolescence (k = 10). Author(s) and publication year Sample sizea Time intervalc Mean ageb M (SD) Setting Assessment of PDs and PD traits Main outcome PD Construct Type of stability Type of PD Instrument Bernstein et al., 1993 (19) 733 24 16.30 (2.8) Community Categories Diagnostic Any PD SCID-II Most PD diagnoses did not persist over time; subjects with PDs identified earlier remained at elevated risk for receiving a PD again at follow-up Bornovalovaet al., 2009 (20) 1,118 120 NR Community Symptoms Mean-level BPD MPQ-BPD Significant mean-level decline from age 14 to 24 Chanen et al., 2004 (21) 96 24 16.10 (0.9) Clinical Categories; symptoms Diagnostic; rank-order (C, D) mean-level DSM-5 PDs SCID-II 74% retained any PD diagnosis over time; low to high cat and dim rank-order; low to high mean-level stability d’Huart et al., 2022 (10) 115 120 15 CW & JJS Categories; symptoms Diagnostic; rank-order (C, D) mean-level DSM-5 PDs SCID-II 47% retained the diagnoses over time; significant increases of small to moderate effect sizes; moderate cat rank-order stability; low to moderate dimensional rank-order stability Greenfield et al., 2015 (22) 204 48 14.6 (1.5) Clinical Categories Diagnostic BPD Ab-DIB 76% retained the diagnosis over time Grilo et al., 2001 (23) 60 24 15.60 (1.7) Clinical Symptoms Mean-level DSM-5 PDs PDE Significant declines for histrionic, narcissistic, dependent, obsessive-compulsive, and passive-aggressive PDs; low to moderate mean-level stability Hamlat et al., 2020 (24) 675 36 11.60 (2.4) Community Symptoms Mean-level; rank-order (D) STPD; HPD; BPD; APD; DPD APD Significant declines of small to medium effect sizes; moderate to high dimensional rank-order stability Johnson et al., 2000 (25) 816 108 13.80 (2.57) Community Symptoms Mean-level; rank-order (D) DSM-5 PDs DISC-I PD symptoms were highest in adolescence and declined linearly to adulthood, although effect sizes were small; low to moderate dimensional rank-order stability; cluster C symptoms seemed to be less stable than cluster A and B symptoms Strandholm et al., 2017 (26) 189 12 16.40 (1.61) Clinical Symptoms Mean-level; rank-order (D) DSM-5 PDs SCID-II Significant declines for most of PD symptoms; low to moderate cat rank-order stability Yen et al., 2013 (27) 99 6 15.3 Clinical Symptoms Rank-order (C) BPD CI-BPD Low cat rank-order stability CW & JJS, child welfare and juvenile justice sample; STPD, schizotypal PD; HPD, histrionic PD; BPD, borderline PD; APD, avoidant PD; DPD, dependent PD; SCID-II, structured clinical interview for DSM-IV personality sisorders; MPQ-BPD, multidimensional personality questionnaire-borderline personality disorder scale; PDE, personality disorder examination; Ab-DIB, abbreviated diagnostic interview of borderlines; DISC-I, diagnostic interview schedule for children; NR, not reported. a Sample size used for the analyses. b Mean age at baseline. c The follow-up interval is displayed in months. Table 3 Longitudinal studies on the course of PDs in adulthood (k = 28). Author(s) and publication year Sample sizea Time intervalc Mean ageb M (SD) Setting Assessment of PDs and PD symptoms Main outcome PD construct Type of stability Type of PD Instrument Alvarez-Tomàs et al., 2017 (28) 41 120 26.90 (6.3) Clinical Categories; symptoms Diagnostic; mean-level BPD SCID-II 50% of participants retained their diagnosis over time; significant decreases in BPD symptoms Black et al. (1995) (29) 26 540 NR Clinical Category Diagnostic ASPD DIS 58% showed (complete) remission 42% showed no remission Bukh et al., 2017 (30) 262 69.6 NR Clinical Categories Diagnostic Any PD SCID-II 72% retained a PD over time Conway et al., 2018 (31) 1,630 60 59.6 Community Symptoms Rank-order (D) BPD SIDP High dimensional rank-order stability over time de Groot et al., 2003 (32) 72 72 NR Clinical Symptoms Rank-order (D) DSM-5 PDs MCMI-II Significant changes for some PD symptoms, whereas others were found to be highly stable Durbin and Klein, 2006 (33) 101 120 32.0 (9.6) Clinical Symptoms Diagnostic; mean-level; rank-order (D) DSM-5 PDs PDE Poor to fair categorical mean-level stability; fair to moderate dimensional mean-level stability; growth curve analyses revealed, however, complex patterns of change in mean scores of PD symptoms Farabaugh et al., 2007 (34) 129 6.5 42.5 (8.91) Clinical Categories Diagnostic Any PD; BPD SCID-II 50% of the participants retained their PD diagnosis over time Hopwood et al. 2013 (35) 266 120 NR Clinical Symptoms Rank-order (D) DSM-5 PDs DIPD-IV Self-reported PD symptoms were substantially higher than clinical interviews PD symptoms both before and after correcting for retest dependability and internal consistency values Kullgren and Armelius, 1990 (36) 41 60 30.90 (7.3) Clinical Categories Diagnostic BPD DIB Diagnostic stability was low and only 56% of all patients retained their diagnosis on follow-up Laptook et al., 2006 (37) 127 120 31.4 Clinical Symptoms Rank-order (C) DPD SCID-II The cat rank-order stability of the diagnosis was fair to moderate Lenzenweger et al., 1999 (38) 250 48 NR Community Symptoms Rank-order (D) DSM-5 PDs IPDE Significant modest declines in PD symptoms over time, while effect sizes were small; high dim rank-order stability Lopez-Castroman et al., 2012 (39) 82 3 38.60 (11.6) Clinical Categories Diagnostic Any PD SCID-II 80% retained the diagnosis over time Loranger et al., 1991 (40) 84 6 29.70 (8.7) Clinical Categories; symptoms Diagnostic; rank-order (C); mean-level Any PD; DSM-5 PDs PDE 73% retained the diagnosis of any PD over time. Notable trend towards fewer symptoms at follow-up than at baseline. Moderate cat rank-order stability Mulder et al., 2010 (41) 149 18 31.6 (NR) Clinical Categories; symptoms Diagnostic; mean-level Any PD; DSM-5 PDs SCID-II 52% retained the PD diagnosis over time; low to moderate diagnostic stability; significant decreases in PD symptoms over time Nestadt et al., 2010 (42) 294 180 47.00 (NR) Community Categories; symptoms Diagnostic; mean-level DSM-5 PDs PDS OCPD exhibited substantial mean-level stability; ASPD, APD, BPD, HPD, STPD exhibited moderate mean-level stability; DPD, NPD, PPD, SPD exhibited low mean-level stability Nysaeter et al., 2012 (43) 14 24 28.90 (6.1) Clinical Categories Diagnostic BPD SCID-II 32% retained the diagnosis over time Paris and Zweig-Frank (2001) (44) 64 324 50.00 Clinical Categories; symptoms Diagnostic; mean-level BPD DIB 7.8% retained the diagnosis over time. Significant decreases in BPD symptoms over time Reichborn-Kjennerud et al., 2015 (45) 2′282 115.2 28.20 (NR) Community Categories; symptoms Diagnostic; rank-order (D) ASPD; BPD SCID-II General declines for both disorders; moderate (BPD) to high (ASPD) dim rank-order stability Riihimäki et al., 2014 (46) 111 60 37.30 (13.7) Clinical Categories Diagnostic BPD SCID-II 57% patients in depressive primary care retained a BPD diagnosis over 5 years Schilders et al., 2017 (47) 776 52.8 NR Community and prison Categories Diagnostic Any PD DIB 30% across settings retained the diagnosis over time; diagnostic stability was higher in prison than in the community setting Silk et al., 1990 (48) 9 27 NR Clinical Categories Diagnostic BPD DIB 56% patients retained a BPD diagnosis over time Trull and Goodwin, 1993 (49) 44 6 28.59 (8.12) Clinical Symptoms Mean-level DMS-5 PDs SCID-II Significant decreases in PD symptoms over time Vaglum et al., 1993 (50) 73 33.6 35.00 (9.00) Clinical Categories Diagnostic; rank-order (C) Any PD SCID-II 56% retained a PD diagnosis at follow-up; high cat rank-order stability Vater et al., 2014 (51) 40 24 30.18 (6.98) Clinical Symptoms Mean-level NPD SCID-II NPD symptoms significantly decreased across time Vergara-Moragues et al., 2013 (52) 200 3 35.01 (7.7) Clinical Symptoms Mean-level PPD; SPD; STPD; HPD; NPD; ASPD; APD; DPD; OCPD MCMI-II Most of PD symptoms in psychoactive substance abuse patients had significantly decreased over time Vrabel et al., 2010 (53) 74 60 29.40 (7.3) Clinical Categories Diagnostic Any PD SCID-II 55% of the patients with longstanding eating disorders retained their initial diagnosis; significant decreases of PD symptoms over time Zanarini et al., 2010 (54) 247 120 26.90 (5.8) Clinical Categories Diagnostic BPD R-DIB 50% of BPD participants achieved a recovery over time PBD, paranoid PD; SPD, schizoid PD; STPD, schizotypal PD; HPD, histrionic PD; NPD, narcissistic PD; BPD, borderline PD; ASPD, antisocial PD; APD, avoidant PD; DPD, dependent PD; OCPD, obsessive-compulsive PD; SCID-II, structured clinical interview for DSM-IV personality disorders; DIS, diagnostic interview schedule; SNAP, schedule for non-adaptive and adaptive personality; MCMI-II, millon clinical multiaxial inventory-II; PDE, personality disorder examination; DIPD-IV, diagnostic interview for DSM-IV personality disorders; DIB, diagnostic interview for borderlines; SIDP, structured interview for DMS-III-personality disorders; IPDE, international personality disorders examination; PDS, personality disorder schedule from the standardized psychiatric examination (SPE); BPD, borderline personality disorder; R-DIB, revised diagnostic interview for borderlines; NR, not reported. a Sample size used for the analyses. b Mean age at baseline. c The follow-up interval is displayed in months. 4.1. Childhood Only two studies to date, namely the studies from Crick et al. (17) and the study from de Clercq et al. (18), have examined the stability of maladaptive personality traits in childhood. While both studies exclusively focused on borderline PD (BPD) traits among community-based, primary school-aged children, they differed regarding the instrument type and the follow-up period, as described in Table 1. While Crick et al. (17) only investigated dimensional rank-order stability, de Clercq et al. (18) investigated both, dimensional rank-order and dimensional mean-level stability. Thus, Crick et al. (17) found only moderate dimensional rank-order stability, while de Clercq et al. (18) found substantial dimensional rank-order stability over time. de Clercq et al’s (18) findings on dimensional mean-level stability indicated that children’s maladaptive trait scores generally decreased as they grow older, with a smaller decline for children who initially had higher levels of maladaptive personality traits. 4.2. Adolescence Overall, ten studies reported data on the stability from adolescence to adulthood (see Table 2). Five studies were from clinical settings (21–23, 26, 27), four studies from community-based samples (19, 20, 24, 25) and one study from a high-risk sample [i.e., young adults with a history of child welfare and juvenile justice placements (10)]. From the studies conducted in clinical settings, two studies (21, 23) were conducted among patients with mixed axis I comorbidities, two studies (22, 27) were conducted among previously suicidal youth and one study (26) was conducted among depressed adolescent outpatients. Three studies (20, 22, 27) focused exclusively on BPD, while the remaining seven studies focused on any PD or most of the DSM-5 PDs. The follow-up period ranged between 6 months (27) and 10 years (10, 20) and four studies (10, 19, 21, 26) used the Structured Clinical Interview for DSM-IV Personality Disorders (SCID-II), while the remaining six studies (20, 22–25, 27) each used different measurement instruments, as presented in Table 2. Most studies focused on PD symptom counts, with only four studies (10, 19, 21, 22) investigating PD categories, three studies (10, 21, 27) reporting data on categorical rank-order stability, seven studies (10, 20, 21, 23–26) reporting data on dimensional mean-level stability, and five studies (10, 21, 24–26) reporting data on dimensional rank-order stability. Findings on diagnostic stability included two studies (21, 22) suggesting substantial stability over time and two studies (10, 19) suggesting only moderate estimates over time. Findings on categorical rank-order stability included two studies (9, 14) indicating moderate categorical rank-order stability for any PD and low to high categorical rank-order stability for individual PD diagnoses, and one study (23) suggesting low categorical rank-order stability for a BPD diagnosis. Findings on dimensional mean-level stability, however, consistently indicated significant decreases for most of PD symptoms over time (20, 21, 23–26). Only one study (10), revealed significant increases for most of PD symptoms over time. The authors concluded that this finding may be explained by the nature of the high-risk sample, as many adolescents in the child welfare and juvenile justice system have experienced severe childhood adversities (e.g., child abuse and neglect) as well as a range of other critical risk factors (i.e., unfavorable parenting practices, low socioeconomic status, childhood psychopathology, self-harming behavior, and youth delinquency) which all have been shown to be significantly associated with the stability of PDs over time. Finally, findings on dimensional rank-order stability revealed highly heterogenous patterns, with three studies (10, 25, 26) ranging from low to moderate, one study (21) ranging from low to high, and one study (24) ranging from moderate to high, depending on PD types. 4.3. Adulthood Overall, 28 studies investigated the stability of PDs in adulthood (see Table 3). Most studies were from clinical settings and only four studies were from community-based samples (29, 31, 38, 42, 44, 45). One study was based on a mixed sample, including both community-based and incarcerated adults (47). Among the studies in clinical settings, seven were conducted among depressed outpatients (30, 33, 34, 37, 39, 41, 48), two were conducted among substance abuse patients (32, 52), and one was conducted among adults with long-standing eating disorders (53). The remaining studies (28, 29, 35, 36, 40, 43, 44, 46, 49, 50) included patients with mixed axis I comorbidities. In addition, nine studies focused exclusively on BPD patients (28, 31, 36, 43, 44, 46, 48, 54), one study focused on BPD and antisocial PD (i.e., ASPD) (45), one study exclusively focused on ASPD (29), and study exclusively focused depressive PD [DPD (37)]; and one study exclusively focused on narcissistic PD (i.e., NPD) (51). The remaining studies either examined “any PD” (30, 34, 39, 50, 53) or DSM-5 PDs (32, 33, 35, 38, 42, 49, 52). The follow-up period varied between 3 months (39) and 45 years (29) and most studies used the SCID-II. In contrast to studies conducted among adolescents, most studies in adulthood focused on PD categories. Thus, 19 studies (28–30, 33, 34, 36, 39–48, 50, 55) reported data on diagnostic stability, revealing highly heterogenous findings, ranging from 7.8% (44) to 80% (39). Three studies (37, 40, 50) reported data on categorical rank-order stability, with two studies (28, 39) indicating moderate and one study (43) indicating high categorical rank-order stability. Nine studies (28, 33, 40–42, 44, 49, 51, 52) reported data on dimensional mean-level stability, consistently suggesting significant declines for most of PD symptoms over time. Finally, six studies (31–33, 35, 38, 45) reported data on dimensional rank-order stability, revealing findings ranging from low to high, depending on the specific type of PD being assessed. 5. Insights from the current literature review Six key findings emerged from the current literature review, which warrant a more detailed discussion. 5.1. Stability estimates in adolescence are comparable to those in adulthood Although research focusing on adolescence has substantially increased over recent years, the number of studies assessing the stability of PDs in childhood and adolescence still appears to be low when compared to studies in adulthood. Part of this may be due to the widespread reluctance to diagnose PDs in adolescence because of the stigma associated with the disorder (56, 57) and the belief that personality in adolescence itself is driven by strong emotions and impulsive behavior (58, 59). Yet recent literature clearly indicates that PDs can be validly and reliably diagnosed prior to the age of 18 years (58–60) and that the stability in adolescence is comparable to that in adulthood. Nevertheless, while maladaptive personality traits can be found as early as childhood, it is reasonable to assume that more severe forms of PDs only become clinically apparent in later adolescence, when individuals have acquired skills to integrate knowledge about themselves and others into a coherent self-identity (61). 5.2. Except for high-risk samples, most PD diagnoses and PD symptoms tend to decrease over time, regardless of age Although most studies largely differed in terms of methodological and conceptual factors, they all seem to converge in the fact that most PD categories (i.e., diagnostic stability) and PD symptoms (i.e., dimensional mean-level stability) decrease over time, while individuals’ rank-ordering (i.e., dimensional rank-order stability) seems to persist. Specifically, studies on the diagnostic stability, overall revealed that many individuals diagnosed with a PD at baseline are likely to not fulfill diagnostic criteria at follow-up. This is most notable, highlighting one of the major shortcomings of the categorical PD system for specific PDs in being based on an arbitrary diagnostic threshold that can easily be met (diagnosis PD) or unmet (no diagnosis PD) by an increase or decrease in a single criterion. This, indeed, favors diagnostic instability, while minor changes in the pathology remain unidentified and the subclinical expression of the individual’s symptoms may remain high (62). Thus, the diagnostic stability of specific PDs appears to be a rather inappropriate measure to assess the stability of PDs over time, as a categorical scaling leads to a substantial loss of information. This shortcoming could be in part compounded by looking at the stability of any PD (including PD NOS) rather than the diagnostic stability of specific PDs. As such, it may be that patients change specific categorical diagnoses but fait to discard the general diagnosis of any PD. Studies on dimensional mean-level stability mostly suggested considerable declines of PD symptoms over time. Although one might think that this is mainly due to treatment effects (63) significant decreases were also found in community-based samples, which suggests a rather natural improvement. While in healthy personality research, mean trait levels tend to change toward increasing maturity in community based settings over time [i.e., decrease in neuroticism, increase in extraversion, agreeableness, and conscientiousness (64)], this might be true for PD traits too. Indeed, the findings of Wright et al. (65), showed that decreases in avoidant PD traits were associated with increases in dominance and warmth and decreases in neuroticism. Studies on dimensional rank-order stability, however, generally indicated moderate to high stability estimates, meaning that individuals who exhibited high levels of a specific PD symptom at one time point also showed relatively high levels of that symptom at a second time point. Taken together, the mean-level of PDs and PD symptoms tends to decrease over time, regardless of participants’ age. Participants’ rank-ordering, however, tends to persist. 5.3. Stability estimates tend to vary with respect to study-specific factors The extent of stability, nonetheless, considerably differed across studies, depending on the PD construct (i.e., categorical diagnoses or dimensional symptoms), the type of stability (i.e., diagnostic, mean-level or rank-order stability), and the specific PD and PD symptom being assessed. In addition, studies differed largely with respect to methodological factors, which yet again, influenced stability estimates. As such, at least six different findings must be emphasized: (a) stability estimates tend to be considerably higher when PDs are assessed dimensionally (i.e., PD symptom counts or PD traits) compared to PDs assessed categorically (PD categories). For instance, the study from Durbin and Klein (33) suggested poor to fair stability estimates for PD categories, while the stability for dimensional PD symptoms were found to be fair to moderate; (b) dimensional rank-order stability estimates seem to be higher than dimensional mean-level stability estimates, meaning that PD symptoms tend to decrease on average, while individual’s rank-ordering in a given sample remains almost the same (33, 66); (c) dimensional stability estimates appear to be higher for self-reported PD symptoms than for interview-based PD symptoms (33, 35, 38, 67). As such, Lenzenweger (38) found smaller 4 years dimensional rank-order stability estimates for interview-based PD symptoms (r = 0.61) than for self-reported symptoms (r = 0.70). Consistently, Durbin and Klein’s (33) stability estimates were 0.49 for interview-assessed symptoms and 0.69 for self-reported symptoms; (d) shorter sampling intervals will generally result in higher stability estimates compared to longer sampling intervals. For instance, dimensional mean-level changes in the Collaborative Longitudinal Personality Disorders Study [CLPS (68)]; were described as “small” at a 2 years follow-up, “medium” at a 4 years follow-up, and “large” at a 10 years follow-up interval; (e) in terms to the type of PD being assessed, cluster B PDs seem to be generally more stable than cluster A and C PDs (25); (f) PD patients in clinical settings seem to attain symptomatic remission more quickly than those from community-based samples. According to Morey and Hopwood (4), one possible reason could be that in clinical samples, participants are often drawn from treatment settings, targeting clinical remission. Therefore, participants in clinical settings tend to show faster declines (i.e., lower stability) compared to other settings. In sum, the extent of stability considerably differs according to the PD type and construct, the type of stability being assessed and several methodological factors, such as the assessment modality, sampling interval, and clinical setting. 5.4. Stability estimates tend to vary with respect to environmental and genetic factors In addition to conceptual and methodological factors, stability estimates, however, also seem to vary as a function of environmental and genetic factors. According to behavioral genetics research, individuals may be genetically predisposed to exhibit more or less stable personality traits. In other words, an individual’s overall score of PD symptoms as well as the extent to which this individual exhibits symptomatic change is strongly heritable (20). Yet individuals evolve within specific environments which may considerably affect stability estimates. As such, the study from Reichborn-Kjennerud and colleagues (45) indicated that the rank-order stability of ASPD and BPD symptoms was largely due to genetic factors, whereas symptomatic change was due to environmental risk factors. Bornovalova and colleagues (20), in contrast, found that stability and change in BPD symptoms were substantially affected by genetic factors and only modestly by environmental factors. However, the authors point out that the strong influence of genetic factors does not mean that environmental factors are unimportant, but rather indicate that the environment, indeed, is likely to influence gene expression, and emphasize the need for interventions to ensure that the individual’s family may serve as a protective factor against the manifestation of pathological traits. 5.5. Symptomatic remission does not equate full recovery Although study findings overall suggest that most PD categories and PD symptoms decrease over the lifespan, it should be kept in mind that a symptomatic remission is not necessarily accompanied by full recovery. Thus, while symptomatic remission is defined as no longer meeting diagnostic criteria for at least 2 years, full recovery is defined as attaining good social and vocational functioning in addition to symptomatic remission. In the McLean Study of Adult Development [MSAD (69)], 34.6% of BPD patients had remitted by the time of the first follow-up (2 years after the baseline assessment), about half (49.5%) had remitted by 4 years follow-up, 69% at 6 years follow-up and 93% had remitted at a 10 years follow-up (2, 54, 70). By the time of the 16 years follow-up assessment, nearly all patients (99%) had experience symptomatic remission and symptom decline stayed relatively stable, with only few patients experiencing symptomatic recurrence (55). However, notably, only half of the patients had achieved significant functional improvements over the 16 years follow-up, with some even experiencing relapse or worsened functioning. Accordingly, the authors conclude that good social and vocational functioning is more difficult to attain than symptomatic remission and, therefore, sustained recovery is much less common than sustained symptomatic remission from BPD. A decrease in PD symptoms is thus not necessarily accompanied by an increase in social and vocational functioning. 5.6. Studies in high-risk samples are scarce Finally, studies investigating the stability of PDs in high-risk samples are surprisingly scarce. Thus, only two studies (10, 47) examined stability estimates in high-risk samples, namely in adolescents placed in the child welfare and juvenile justice system (10) and incarcerated adults (47). This is especially striking given that individuals from high-risk samples are particularly at risk for developing a PD. Consistently, both studies (10, 47) suggested substantial increases in PD diagnoses over time (11, 45), while clinical and community-based studies overall converged in that most PD diagnoses and symptoms decrease over time. 6. Implications Overall, studies suggest that PDs, either assessed categorically or dimensionally, are not as stable as previously assumed. This highlights the need to overcome the clinical assumption that PDs are “enduring,” “pervasive” and “inflexible” over time. This emphasizes that PDs are treatable, and thus, should be assessed and diagnosed prior to the age of 18 in order to provide the best possible outcome later in life. As a consequence, patients as well as clinicians may be cautiously optimistic about the prognosis of a PD. In addition, if PDs and PD symptoms are not as stable as previously thought, this raises the question whether it is still appropriate to consider stability as a central feature of PDs? In other words, is it still reasonable to refer to a PD or PD symptoms, if the concept itself depends on numerous conceptual, methodological, genetic, and environmental factors? Or is it rather the general level of personality functioning (i.e., self and interpersonal functioning), which is conceptually separated from PD categories and symptoms, that actually determines a PD? This issue, in turn, emphasizes the current shift to more dimensional conceptualizations, as defined in the AMPD or ICD-11. In fact, both models introduce a radical change in the structure and diagnosis of PDs, by conceptualizing PDs as core impairments in self-and interpersonal functioning, amplified by a severity ranking and specific trait specifiers related to negative affectivity, detachment, dissociality (i.e., antagonism in the AMPD), disinhibition, and/or anankastia in the ICD-11 and psychoticism in the AMPD. We suggest that moving away from PD categories and PD symptoms helps clinicians to perceive the patient as a whole, by refocusing on the original meaning of personality, that is the subjective experience of what it means to be human (71). This may help to not only see if patients suffer, but also how they suffer. While the classification of severity may help inform clinical prognosis and intensity of treatment, the classification of trait specifiers may help to identify individual problems, resulting in more individualized, tailor-made treatments (72, 73). To this date, the literature currently lacks data about the stability of the general level of personality functioning. Although we have reasons to think that it may be more stable, e.g., (12, 13), this remains to be proven. We therefore suggest that future research should focus more intensively on personality functioning and specific trait expressions in order to determine whether AMPD’s and ICD-11’s new conceptualizations clarify the issue of stability over time. Specifically, studies should investigate the course and outcome of personality functioning and pathological personality traits from childhood to late adulthood. Thereby, research should increasingly rely on dimensional assessments and longer follow-up intervals. Future work on the etiological origins of these constructs and the mechanisms by which these constructs evolve over time, will be of great importance. Moreover, future research needs to address methodological factors to prevent unnuanced responses to the complex notion of stability. In fact, researchers still often use the general term “stability” without being explicit regarding the type of stability they are referring to. This is particularly problematic as different types of stability can vary substantially as pointed out in the present review. In addition, future studies should incorporate more sophisticated sampling and statistical procedures to overcome possible limitations. In particular, studies should focus on multi-wave study designs, including multiple measurement points, in order to analyze the shape of each person’s individual trajectory and distinguishing true change from measurement error (74). Furthermore, studies of high-risk samples, especially in childhood and adolescence, may be crucial as these children and adolescents are particularly at risk of developing maladaptive personality traits and PD prevalence rates among these samples are alarmingly high. Finally, and most importantly, upcoming research should address genetic, contextual, and situational factors that may influence the course of PDs or personality functioning over the lifespan. After all, while the direction of change is known, the causes of change remain unclear. 7. Conclusion In recent decades, research on the stability of PDs has considerably increased, yet it remains a much-debated topic as it is foremost a conceptual and methodological endeavor. This narrative review, however, has highlighted key findings from the current literature, suggesting comparable stability estimates in adolescence and adulthood, with considerable improvement over time. Future work may, eventually, determine whether the new conceptualization will clarify some of the issues related to the stability of PDs. Nevertheless, it should be acknowledged that a symptomatic remission is not necessarily accompanied by a full recovery, with most PD patients never managing to fully participate in society, despite considerable remission. Understanding the process of change is thus particularly important, in order to identify protective factors, that potentially might mitigate long-term impairments. Taken together, these findings challenge our current understanding of PDs in terms of disorders and symptoms and argue instead in favor of the AMPD and ICD-11 reintroducing the idea of self and interpersonal functioning as the core feature of PDs. This might enable clinicians to perceive the patient as a whole, by identifying individual problems, which, could, ultimately, contribute to more personalized and tailor-made treatments. Author contributions DH and BB contributed to conceiving and designing the present manuscript. DH and SS conducted the literature search. DH wrote the first draft of the manuscript. SS, DB, MB, CB, MS, KS, and BB commented on an earlier draft of this article and supervised the entire process. All authors contributed to the article and approved the submitted version. Funding DH was funded by an individual PhD fellowship from the Fonds National de la Recherche du Luxembourg (FNR). Conflict of interest The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer YC declared a past co-authorship with one of the authors BB to the handling editor. Publisher’s note All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. 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