Introduction
Psychopathy is a multifaceted syndrome often described as a constellation of affective (e.g., lack of remorse or guilt, shallow affect, callous/lack of empathy), interpersonal (e.g., glib/superficial charm, grandiose self-worth, manipulative), behavioral and antisocial (e.g., poor behavior control, impulsive, inability to accept responsibility for one’s actions) features [
17,
38]. Psychopathy is related to a range of negative and dysfunctional outcomes including substance use, criminal behavior, psychopathology (e.g., borderline personality disorder) [
21,
40,
42,
44,
56], and social maladjustment such as lower educational performance, unemployment and poor social relationships [
4,
19,
44,
71]. The prevalence of psychopathy has been estimated to be between 0.6 and 4 % in the general population, with a higher proportion of males to females [
65,
75]. Despite the low prevalence, these individuals are believed to account for a large portion of all serious crimes and their recidivism rate is higher than for other offenders [
8,
38,
56]. Furthermore, psychopathy is considered to be a neurodevelopmental disorder rooted early in life [
22,
33,
35,
36,
41,
52]. Identifying individuals with psychopathic personality traits early in development is, therefore, crucial for intervention efforts, especially since these traits have been linked to early engagement in criminal activities (e.g., [
39,
58,
62]). To date, there has been very limited research on the genetic and environmental etiology of psychopathic personality traits in young children. This study aimed to bridge this research gap by investigating the extent of genetic influences on these traits in boys and girls and to investigate whether shared environmental influences also play a significant role.
Recent twin studies report that heritable factors have a moderate to high influence, non-shared environmental factors have a small to moderate influence, and shared environmental factors have little or no importance explaining the variance in psychopathic personality (for reviews: [
66,
72]). This has been found among adolescents (e.g., [
5,
9,
25,
30,
47,
48,
64,
69,
70]), as well as among adults (e.g., [
5,
10,
11,
44]). However, a more mixed pattern has been found across the few studies that have included children (i.e., participants 12 years of age or younger; see Table
1 for a summary). Please also note that the majority of the studies summarized in Table
1 have only examined the affective (or callous–unemotional) traits. For example, an early study by Viding et al. [
76] using DeFries–Fulker extreme analysis showed that the heritability of callous–unemotional traits was 67 % in a sample of 7-year-old twins. Furthermore, Bezdjian et al. used the Child Psychopathy Scale [
51] to assess psychopathic personality traits in a set of 9–10 year old twins. The affective–interpersonal factor was primarily influenced by genetic factors with slight sex differences (boys 64 %; girls 49 %), with the non-shared environment contributing 36 % in boys and 44 % in girls. Similarly, the impulsive–antisocial factor was primarily influenced by genetic factors (boys 46 %; girls 58 %), with the non-shared environment contributing 53 % in boys and 37 % in girls. There were little and non-significant influence from the shared environment [
6]. Ficks et al. included children as young as 4 years in their study (age range 4.4–17.8 years, age-corrected analysis) and the antisocial process screening device [
31] was used to assess psychopathic personality traits. For callous–unemotional traits, genetic influences explained 49 % of the variance, shared environment 19 % and the non-shared environment 32 %; for narcissism, genetic influences explained 63 % and the non-shared environment 37 %; and for impulsivity, additive and non-additive genetic influences contributed 61 % in boys and 74 % in girls, with the non-shared environment contributing the remaining variance.
Table 1
Genetic (A), shared environmental (C) (or dominant, D), and non-shared environmental (E) estimates for psychopathic personality traits in children (i.e., participants 12 years of age or younger)—a summary from previous twin studies
| RFABa (1219 twins) | Child Psychopathy Scale (CPS) [ 50] | Caregiver rated (>90 % biological Mothers) | 9–10 | Affective–interpersonal factor (boys) | 0.64 (0.49 to 0.72) | 0 (0.00 to 0.11) | 0.36 (0.28–0.47) |
Affective–interpersonal factor (girls) | 0.49 (0.21 to 0.65) | 0.06 (0.00 to 0.30) | 0.44 (0.35–0.56) |
Impulsive–antisocial factor (boys) | 0.46 (0.22 to 0.58) | 0.01 (0.00 to 0.195) | 0.53 (0.41–0.66) |
Impulsive–antisocial factor (girls) | 0.58 (0.25 to 0.70) | 0.04 (0.00 to 0.34) | 0.37 (0.29–0.48) |
| Georgia Twin Studyc (885 twin pairs) | Antisocial process screening device [ 31] | Mother rated | 4.4–17.8 | Callous–unemotional | 0.49d
| 0.19 | 0.32 |
Narcissism | 0.63 | | 0.37 |
Impulsivity (boys) | 0.21 | 0.40 | 0.39 |
Impulsivity (girls) | 0.26 | 0.48 | 0.26 |
| TEDSb (612 + 234 + 210 twins) | Three antisocial process screening device [ 31] and four Strengths and Difficulties Questionnaire [ 37] items were used to assess callous–unemotional traits | Teacher rated | 7 | Elevated levels of callous–unemotional traits | 0.67e (0.47 to 0.87) | 0.06 (−0.23 to 0.35) | |
Elevated levels of antisocial behavior and callous–unemotional traits | 0.81 (0.50 to 1.12) | 0.05 (0.00 to 0.72) | |
Elevated levels of antisocial behavior w/o callous–unemotional traits | 0.30 (−0.10 to 0.70) | 0.34 (−0.40 to 1.08) | |
| TEDSb (352 + 234 twins) | All items from callous–unemotional dimension of the antisocial process screening device [ 31] and two items from the Strengths and Difficulties Questionnaire [ 37] were used to assess callous–unemotional traits | Teacher rated | 7 | Callous–unemotional traits and elevated levels of antisocial behavior | 0.68e (0.42 to 0.95) | 0.00 (−0.82 to 0.18) | |
Callous–unemotional traits w/o elevated levels of antisocial behavior | 0.80 (0.51 to 1.03) | 0.00 (−77 to 0.23) | |
| TEDSb (140 probands in 88 twin pairs, and 174 probands in 144 twin pairs) | All items from callous–unemotional dimension of the antisocial process screening device [ 31] and two items from the Strengths and Difficulties Questionnaire [ 37] were used to assess callous–unemotional traits | Teacher rated | 9 | Antisocial behavior and callous–unemotional traits | 0.75e (0.45 to 1.06) | 0.00 (−1.63 to 1.27) | |
Antisocial behavior only | 0.53 (0.13 to 0.92) | 0.00 (−0.83 to 0.64) | |
Antisocial behavior, hyperactivity and callous–unemotional traits | 0.71 (0.24 to 1.18) | 0.00 (−1.66 to 1.36) | |
Antisocial behavior and hyperactivity | 0.36 (−0.14 to 0.86) | 0.00 (−0.76 to 0.71) | |
| TEDSb (627 + 119 twins) | Three antisocial process screening device [ 31] and four Strengths and Difficulties Questionnaire [ 37] items were used to assess callous–unemotional traits | Teacher rated | 7 | Elevated levels of callous–unemotional | 0.75e (0.58 to 0.92) | −0.02 (−1.22 to 2.38) | |
Elevated levels of callous–unemotional and Anxiety | 0.66 (0.33 to 0.95) | 0.05 (−1.62 to 2.28) | |
| TEDSc (9462) | Three Antisocial Process Screening Device [ 31] and four Strengths and Difficulties Questionnaire [ 37] items were used to assess callous–unemotional traits | Teacher rated | 7, 9, 12 | Stable high (boys) (girls) | 0.78 (0.42 to 0.88) | 0.01 (0.00 to 0.35) | 0.21 (0.12–0.34) |
0.00 (0.00 to 0.57) | 0.75 (0.35 to 0.90) | 0.25 (0.07–0.48) |
Increasing (boys) (girls) | 0.58 (0.12 to 0.72) | 0.03 (0.00 to 0.41) | 0.39 (0.28–0.53) |
0.26 (0.00 to 0.70) | 0.47 (0.08 to 0.74) | 0.27 (0.16–0.43) |
Decreasing (boys) (girls) | 0.61 (0.35 to 0.72) | 0.02 (0.00 to 0.23) | 0.37 (0.28–0.49) |
0.54 (0.23 to 0.85) | 0.26 (0.00 to 0.53) | 0.20 (0.13–0.29) |
Stable low (boys + girls) | 0.68 (0.52 to 0.81) | 0.08 (0.00 to 0.21) | 0.24 (0.19–0.30) |
In sum, studies examining the genetic and environmental etiology of psychopathic personality traits in young children are scarce. The few studies that do exist have produced diverse findings showing a moderate to strong heritability; the role of the shared environment on these traits is mixed, and whether or not the genetic and environmental estimates vary across sex is unclear. Using data from a population-based sample of Swedish 5-year-old twins this study had the following goals: (1) to examine the genetic and environmental etiology of the three psychopathic personality dimensions—grandiose–deceitful, a callous–unemotional, and impulsive–need for stimulation-assessed with the Child Problematic Traits Inventory (CPTI; [
13]), which was designed to be used specifically among young children; (2) to examine whether the genetic and environmental etiology of the three psychopathic personality dimensions is comparable in boys and girls; and (3) to examine how much of the phenotypic correlation among these dimensions that are accounted for by genetic and environmental influences.
Results
Descriptive statistics and correlations
There were significant mean differences between boys and girls, with boys having higher mean values for grandiose–deceitful, callous–unemotional, and impulsive–need for stimulation. The pattern of the twin correlations indicates that genetic and shared environmental influences are important for the three psychopathic personality dimensions, Table
2.
Univariate genetic analysis
Univariate model-fitting results for grandiose–deceitful, callous–unemotional, and impulsive–need for stimulation are displayed in Table
3. A low DZ twin correlation (in boys for grandiose–deceitful; boys and girls for impulsive–need for stimulation, Table
2) may be due to non-additive genetic effects, such as epistasis or dominance [
54]. A model estimating additive genetic (A) effects, non-additive genetic (D) effects and non-shared environmental (E) effects was, therefore, first tested. However, the full ACE model (Model 2 in Table
3) was found to fit better than the ADE model (grandiose–deceitful: AIC 768.632, BIC −2210.855; impulsive–need for stimulation: AIC 694.695, BIC −2247.824).
Table 3
Univariate genetic results and parameter estimates for the psychopathic personality dimensions grandiose–deceitful, callous–unemotional and impulsive–need for stimulation at age 5, teacher ratings
Grandiose–deceitful |
1. Saturated model | 3109.314 | 1172 | 765.314 | −2194.920 | | | | | | |
2. ACE boys ≠ girls | 3116.395 | 1181 | 754.395 | −2220.173 | 7.081 | 9 | 0.629 | | | |
3. ACE boys = girls | 3120.357 | 1184 | 752.357 | −2227.789 | 11.043 | 12 | 0.525 | 0.57 (0.39–0.75) | 0.17 (0.001–0.32) | 0.26 (0.21–0.33) |
Callous–unemotional |
1. Saturated model | 3018.635 | 1172 | 674.635 | −2240.259 | | | | | | |
2. ACE boys ≠ girls | 3023.728 | 1181 | 661.728 | −2266.506 | 5.093 | 9 | 0.826 | | | |
3. ACE boys = girls | 3028.018 | 1184 | 660.018 | −2273.959 | 9.383 | 12 | 0.670 | 0.25 (0.10–0.40) | 0.48 (0.35–0.60) | 0.27 (0.22–0.33) |
Impulsive–need for stimulation |
1. Saturated model | 3035.396 | 1172 | 691.396 | −2231.879 | | | | | | |
2. ACE boys ≠ girls | 3049.638 | 1181 | 687.638 | −2253.552 | 14.242 | 9 | 0.114 | | | |
3. ACE boys = girls | 3050.872 | 1184 | 682.872 | −2262.532 | 15.476 | 12 | 0.216 | 0.74 (0.59–0.86) | 0.09 (0.00–0.23) | 0.17 (0.14–0.21) |
The full ACE model described the data better than the baseline saturated model (Model 2, grandiose–deceitful: χ
2 = 7.081, df = 9, p = 0.629, callous–unemotional: χ
2 = 5.093, df = 9, p = 0.826, impulsive–need for stimulation: χ
2 = 14.242, df = 9, p = 0.114); Model 2 also had smaller AIC and BIC. A model constraining genetic and environmental components to be equal in boys and girls provided a better fit than the full ACE model (Model 3, grandiose–deceitful: χ
2 = 3.962, df = 3, p = 0.266, callous–unemotional: χ
2 = 4.29, df = 3, p = 0.232, impulsive–need for stimulation: χ
2 = 1.234, df = 3, p = 0.745). Genetic influences accounted for 57, 25, and 74 % of the phenotypic variance for grandiose–deceitful, callous–unemotional and impulsive–need for stimulation, respectively; shared environmental factors accounted for 17, 48, and 9 % (n.s.), and non-shared environmental factors (including error) accounted for the remaining variance, 26, 27, and 17 %, respectively.
Bivariate genetic analysis
A bivariate Cholesky decomposition was next fit to data. The Cholesky decomposition fit the data better than a saturated model (
χ
2 = 76.143,
df = 69,
p = 0.260). Similar to the univariate analyses, the genetic and environmental variance components could be constrained to be equal in boys and girls (
χ
2 = 21.157,
df = 18,
p = 0.272). The phenotypic correlations were moderate to high across the three psychopathic personality dimensions (Table
3). The phenotypic correlations between grandiose–deceitful, callous–unemotional, and impulsive–need for stimulation were primarily accounted for by genetic and shared environmental influences (Table
4).
Table 4
Proportion of the phenotypic correlations between grandiose–deceitful, callous–unemotional, and impulsive–need for stimulation accounted for by genetic (A), shared environmental (C) and non-shared environmental (E) factors
Grandiose–deceitful/callous–unemotional | 0.66 | 0.25 (0.05–0.43) | 0.52 (0.36–0.67) | 0.23 (0.17–0.32) |
Grandiose–deceitful/impulsive–need for stimulation | 0.61 | 0.56 (0.40–0.72) | 0.26 (0.12–0.40) | 0.18 (0.12–0.24) |
Callous–unemotional/impulsive–need for stimulation | 0.54 | 0.39 (0.20–0.59) | 0.44 (0.26–0.60) | 0.17 (0.11–0.24) |
Discussion
This study aimed to investigate the genetic and environmental sources among three psychopathic personality dimensions, grandiose–deceitful, callous–unemotional and impulsive–need for stimulation in a community sample of 5-year-old children assessed by teachers. There are three main points of interest for discussion in this study. First, familial influences (i.e., genetic and/or shared environment) explained the majority of variance in grandiose–deceitful, callous–unemotional and impulsive–need for stimulation. Second, no sex differences were found in the genetic and environmental variance components. Third, the proportions of the phenotypic correlations among these dimensions were mainly mediated by genetic and shared environmental influences.
Similar to Ficks et al. [
25], our univariate analyses indicated that genetic and shared environmental influences primarily explained the variance in the callous–unemotional dimension, and that a large genetic influence was important for impulsive–need for stimulation. We also found that genetic and shared environmental influences explained the variances in grandiose–deceitful, whereas Ficks et al. found that mainly genetic influences were important for narcissism. This discrepancy in findings between our study and Ficks et al. could partly be explained by methodological differences in that we were using the CPTI rated by teachers and they were using the antisocial process screening device rated by mothers. As genetic influences on psychopathic personality traits may vary across the ways in which these traits are measured, in terms of both informant and instrument used [
69], more research examining the genetic and environmental etiology of these traits in early childhood is warranted. Also, Ficks et al. [
25] age-corrected their data (age range 4.4–17.8 years), whereas we used a sample of 5-year-old twins.
Further, callous–unemotional traits have previously received attention [
25,
32,
41], and recently a callous–unemotional-based specifier for the diagnosis of conduct disorder has been added in the fifth edition of the diagnostic and statistical manual of mental disorders [
2]. Our finding of a moderate genetic (25 %) influence and higher shared environmental (48 %) influence on the callous–unemotional dimension is in sharp contrast to findings by Viding et al. [
76] who found a high heritability (67 %) for antisocial behavior in the presence of callous–unemotional traits as reported by teachers in a sample of 7 year old twins, and no influence from the shared environment. Thus, we found that both genetic and shared environmental factors contributed to callous–unemotional traits at age five. Shared environmental risk factors may include family related factors (e.g., neglect, prenatal stressors) or contextual factors in the surrounding community [
53,
68]. Our finding agree with prior work linking environmental factors to callous–unemotional traits and studies suggesting that interventions focusing on environmental stimuli may be effective in reducing callous–unemotional traits (for a review: [
34]). The moderate genetic influence in our sample for callous–unemotional traits might also be related to heterogeneity within these traits, with subgroups showing differences in behavioral and physiological measures of anxiety and fear reactivity (e.g., [
23,
24]).
The three dimensions grandiose–deceitful, callous–unemotional and impulsive–need for stimulation were all moderately correlated. The proportions of these phenotypic correlations were mainly accounted for by genetic and shared environmental influences. Again, these findings provide support for the importance of both genetic and shared environmental influences in psychopathic personality traits in young children.
The significant shared environmental influences in particular for callous–unemotional traits identified in our sample of 5-year-old twins are of great importance. Typically, a pattern of decreasing shared environment and a concomitant increase in heritability over the course of development is found; this has been reported for several phenotypes including personality traits, cognitive abilities, and aggression [
59]. It will be interesting to follow the twins included in this study across development to see if a similar pattern will emerge for the shared environment on psychopathic personality traits. Then again, the bulk of literature on psychopathy has shown little or no influence of the shared environment (e.g., [
66,
72]); however, the majority of previous research has been conducted on adolescent or adult twin samples. Of note, twin studies typically have low power to detect shared environmental influences relative to genetic influences. Shared environmental influences can also be confounded with for example the effects of assortative mating or passive gene-environment correlation (
rGE) [
46]. Thus, part of the shared environment we found could be explained by the fact that the same teacher was rating both twins in a pair [
3]. In our case, 97 % of the participating twin pairs went to the same pre-school class, and 85 % were rated by the same teacher. Studies of children typically rely on parent or teacher reports; it is, therefore, possible that the shared environment found in these studies and in our study is partly an artifact of rater bias. This suggests that future studies are needed examining how genetic and environmental factors influence psychopathic personality traits in children, and it will be interesting to see if they can replicate our finding of a shared environmental component.
We also found higher mean values for psychopathic personality traits in boys than girls across all three psychopathic personality dimensions, indicating that these traits are somewhat more prevalent among boys than girls. Higher mean values for psychopathic personality traits have also been found among males than females across incarcerated and community samples [
75]. However, no differences in the magnitude in genetic and environmental variance components were found across boys and girls and the variance components could be constrained to be equal. This finding is in contrast to Bezdjian et al. [
7], who found significant sex differences across 9–10 year old boys and girls, with the affective–interpersonal factor showing higher heritability in boys and the impulsive–antisocial factor showing higher heritability in girls. Similarly, Ficks et al. [
25] found sex differences in the Impulsivity dimension, with a higher heritability in girls. Our findings suggest that despite sex differences on a mean level, the underlying genetic and environmental etiology of these traits appears to be similar for both boys and girls. This would in turn indicate that there are specific circumstances (biological) or experiences (social, environmental) that may lead to greater expression of psychopathic personality traits in boys. Future research needs to determine which specific factors that contributes to the sex difference in prevalence.
Limitations
A few limitations in this study must be considered when interpreting these findings. First, we examined the genetic and environmental influences on psychopathic personality traits in a community sample of young twins. Our results may not be generalizable to children in clinical settings. We only had one time point, future research need to investigate how genetic and environmental factors influence change in these traits from early childhood through adolescence and whether the shared environment that we found will decrease across development. There are several assumptions related to the classical twin design [
59], for example, the heritability estimate is time and population specific. A more detailed discussion of these and other assumption in the twin design in relation to psychopathology can be found elsewhere [
67].