Emerging adulthood is defined as the transition from adolescence to adulthood and is considered a challenging and important developmental period, ranging from 18 to 29 years of age (Arnett, 2000). This period is characterized by rapid changes in social context, greater freedom, and less social control compared to adolescence (Henin and Berman, 2016). Recent studies show emerging adulthood is a relevant period regarding onset and changes in offending behavior, in part because the peak of the age-crime curve lays between the ages of 21 and 25 (Craig and Piquero, 2015).

Studies assessing normal brain development show that gray matter decreases and white matter increases may underly the reduction in risk taking and the increases in self-regulation, future orientation, and planning that take place during emerging adulthood (Taber-Thomas and Pérez-Edgar, 2015). Whereas, brain development is deemed crucial for the relationship between adolescence and crime (Steinberg, 2009), there is little research specifically investigating brain abnormalities related to criminal behavior in emerging adulthood.

A recent review on the relationship between white matter integrity and antisocial behavior (including aggression and psychopathic/callous-unemotional traits) across the lifespan underlines this limitation in the literature. Results show antisocial behavior is negatively associated with white matter integrity across a range of white matter tracts in adults, but that studies in adolescent populations are mixed and inconclusive, whereas studies in emerging adulthood are absent all together (Waller et al., 2017). These contradicting results could not be explained by differences in sample type, age, or analysis strategy, and future studies are advised to conduct studies in a narrower age range.

White matter integrity can be assessed with diffusion tensor imaging, measuring the displacement of water molecules within white matter tracts. The structure of the axonal cell membranes and myelin sheath hinder the diffusion of water molecules in other directions than along the fiber tract (Pierpaoli and Basser, 1996). Four often used measures for white matter integrity are fractional anisotropy (FA),mean diffusivity (MD), radial diffusivity (RD) and axial diffusivity (AD), reflecting diffusion of water molecules along the fiber tract (FA), overall diffusion (MD), average diffusivity along the other two minor axes (RD) and the magnitude of diffusion parallel to fiber tracts (AD) respectively (Beaulieu, 2002). Higher FA or AD and lower MD or RD values reflect better white matter integrity, associated with denser axonal packing, increased coherence of axon orientations, and greater myelination (Beaulieu, 2002).

The current study assesses differences in white matter integrity between a relatively large sample of delinquent young adults (age 18–28) and age-matched healthy controls. Within the sample of delinquent young adults, the relationships between white matter integrity, aggression, psychopathic traits, and cannabis use are assessed.

The current study assessed possible differences between delinquent emerging adults and age-matched healthy controls in white matter integrity, but no group differences in FA, MD, RD or AD values were found. Even though differences in self-reported aggression—especially proactive aggression—cannabis use and interpersonal psychopathic traits were found between groups, these were not related to white matter integrity within the delinquent emerging adults. No differences were found for self-reported total, behavioral or affective psychopathic traits or reactive aggression.

Together with the inconsistent findings in the review by Waller et al. (2017), our results may suggest that (1) white matter integrity differences are unrelated to antisocial behavior in emerging adults, and/or that (2) white matter differences between delinquent populations and controls may only arise later in life. A later offset of white matter differences might be related to a number of factors, including known lifestyle differences between these groups [see Semenza (2018)]. Studies show that poor diet, lack of exercise, and drug/alcohol use decrease white matter integrity (Gow et al., 2012; Kokubun et al., 2020). Previous studies in substance dependent adolescents, however, indicate that white matter integrity is affected early on (Baker et al., 2013), contradicting the above explanation. Little is known about the effect of these lifestyle choices on white matter integrity in emerging adulthood—most studies were conducted in elderly subjects. Whether such lifestyle choices would mediate the relationship between delinquency and white matter integrity differences found in adults is an interesting topic for further research.

Another explanation for a delayed onset of white matter differences is the higher risk of traumatic brain injury (TBI) in delinquent populations (Durand et al., 2017). TBI is known to affect white matter integrity (Croall et al., 2014), may be present in about 46% of delinquent adults, and may be a risk factor for crime (Williams et al., 2018). TBI in delinquent populations is most often acquired due to traffic accidents and interpersonal violence (Durand et al., 2017), both of which are likely to peak during emerging adulthood (Craig and Piquero, 2015). Unfortunately, no data on TBI were collected for our current sample.

Surprisingly, the delinquent group and the matched controls did not differ on the affective and behavioral components of the YPI, nor on self-reported reactive aggression. This could be a chance finding, which could hinder interpretation of our group comparisons—but not the regression analysis for the delinquent sample. However, analysis of internal validity, construct validity and a factor analysis revealed that the YPI is a valid instrument in our sample [see Zijlmans et al. (2019) and Supplementary Information 2].

In addition to the points outlined above, delinquent young adults are often characterized by a multitude of problems, including addiction, mental health issues, and a poor social network (Zijlmans et al., 2021). Although our sample size of n = 22 should provide enough power to identify group differences in white matter integrity between the delinquent group and matched controls (De Santis et al., 2014), the heterogeneity within the delinquent group could be associated with variation in underlying white matter differences in location and integrity. This could obscure significant results from a group analysis. Future studies should investigate this variation by taking these multiple problems into account in group analyses, analyzing more heterogenous subgroups and using larger samples for sufficient power, while also increasing the number of controls. Assessing which lifestyle choices may mediate the relationship between white matter integrity and antisocial behavior in delinquent populations could also add to this growing body of research.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by Medical Ethical Committee of the Amsterdam University Medical Centers. The patients/participants provided their written informed consent to participate in this study.

JZ and RM collected the data. JJ performed the statistical analysis and wrote the first draft of the manuscript. JJ and JZ wrote sections of the manuscript. All authors contributed to manuscript revision, read, and approved the submitted version. All authors contributed to the conception and design of the study.

This research project is funded by the De Verre Bergen Foundation. De Verre Bergen Foundation is a venture philanthropy organization that aims to build a better Rotterdam through substantial investments in innovative, impactful social ventures. The financer was not involved in the design of the study nor the drafting of the manuscript. Furthermore, the financer was not involved in the process of data collection, analysis, and interpretation.

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.

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