Background

Violence has become a critical global concern, with homicides alone accounting for more than 475 000 deaths globally. This figure is particularly alarming considering that violence manifests in various forms beyond homicide, including child maltreatment and sexual violence. The number of victims continues to rise across these categories. Women and children are disproportionately affected by violence, making them vulnerable groups [1]. For example, the United Nations Entity for Gender Equality and the Empowerment of Women stated that in 2023, approximately 51 100 women and girls were killed by their intimate partner or other family members [2].

Despite efforts to raise awareness in several countries and the commitment to fight against gender-based violence by working under the Sustainable Development Goal (SDG) 5 to achieve gender equality and empower all women and girls, the prevalence of violence against women continues to rise [1].

Violent behavior is influenced by environmental, genetic, and epigenetic factors [3]. The present article focuses on the genetic component, specifically analyzing the bibliographic evidence associating Y and X chromosome genetics with violent behavior.

Chromosomes

Chromosomes are structures within the nucleus of body cells. Humans typically have 23 pairs of chromosomes (46 in total). The 23rd pair is the sex chromosomes, which determine an individual’s biological sex. Generally, biological females have two X chromosomes, with one of them randomly inactivated, while biological males have one X and one Y chromosome [3]. Notably, the X chromosome contains about 800 protein-coding genes, while the Y chromosome has around 60 [3].

The X chromosome´s association with violent behavior

X-linked polymorphisms, especially in the MAOA gene, have been associated with violent behavior. This gene encodes the monoamine oxidase A enzyme, responsible for the catabolism of neurotransmitters such as serotonin and dopamine. Additionally, a variable number of tandem repeats polymorphism within the MAOA gene has also been correlated with increased violent tendencies. The variable number of tandem repeats polymorphism consists of 30 base-pair repeat (R) sequence, with alleles ranging from 2R to 6R [4].

Based on their genetic structure, these variants are categorized into high transcriptional activity (MAOA-H) alleles (3.5R-4R), which increase MAOA expression, and low transcriptional activity (MAOA-L) alleles (3R), which, in turn, decrease gene expression. Variants associated with a reduced MAOA expression level have been linked to aggression, anger, and violent behavior [4].

Notably, research on the MAOA variable number of tandem repeats has primarily identified an association with aggression in men. This is because the random inactivation of one X chromosome in females complicates the identification of such associations.

Other studies have also linked other MAOA variants, such as single-nucleotide polymorphisms, to violent behavior. These involve single base change in the DNA sequence. For instance, several single-nucleotide polymorphisms, including rs5906957, rs909525, rs6363, and rs2064070, have been associated with physical aggression in boys or anger in adult men [5].

Chromosomal number abnormalities, such as Jacobs’ syndrome (47XYY), have also been associated with aggressive behavior in some studies. However, these studies lacked sufficient statistical evidence, and recent research indicates that there is no significant evidence linking this syndrome to an increased predisposition for violent behavior [6].

Furthermore, the relationship between people with fragile X syndrome and aggressive behavior has been well-documented. For instance, Wheeler A.C. et al. 2015 found that more than 90% of male and female subjects with fragile X syndrome have displayed violent behavior. Notably, in male individuals, 30% of such violent episodes resulted in injuries [7].

The Y chromosome´s association with violent behavior

One of the most important genes located on the Y chromosome is SRY (Sex determining region Y). The protein encoded by this gene functions as a transcription factor that binds to specific DNA regulatory regions such as promoters and enhancers [8]. The principal function of SRY is to direct embryonic gonads to develop as testes, which will secrete hormones that masculinize other male characteristics, including physiology and behavior [8,9].

Interestingly, SRY is also expressed in other tissues, including the heart, lungs, adrenal glands, and brain. This suggests that SRY has additional roles beyond sex determination [9]. In the brain, it has been linked to the “fight or flight” response by increasing catecholamine release and inhibiting the monoamino oxidasa A, thereby stimulating aggression. Notably, the authors hypothesize that during early human evolution, when males were hunters competing for resources, SRY may have evolved not only to determine sex but also to influence male behavior, fostering aggression to improve survival [9].

However, studies investigating the role of the Y chromosome in aggression have yielded conflicting results, supporting or rejecting its role in violent behavior. Further research is needed to clarify the association between the Y chromosome and aggressive behavior. Moreover, genes and single-nucleotide polymorphisms located on other chromosomes may also be associated with violence. For instance, IPO13, PCDH7, and ST3GAL3 were linked to childhood aggression in genome-wide association studies [10].

Future perspectives

Human behavior is shaped by genetic, epigenetic and environmental factors, making its study inherently complex. This complexity underscores the intricate interplay of biological and external influences on behavioral traits. Genome-wide association studies offer a potential approach to identifying single-nucleotide polymorphisms associated with violent behavior, thereby enhancing our understanding of its genetic basis. Furthermore, genome-wide association studies can be complemented by other genetic analyses, including gene expression studies, next-generation sequencing, and karyotyping, among others. Integrating these approaches could lead to a comprehensive understanding of the genetic component underlying violent behavior.

Conclusions

This paper has examined the potential roles of the X and Y chromosomes in violent behavior, incorporating findings from genetic and evolutionary perspectives. Evidence suggests that men, who statistically exhibit higher levels of aggression, may partly present this tendency due to genetic influences, such as polymorphisms in the MAOA gene on the X chromosome and the behavioral effects of the SRY gene on the Y chromosome. These genetic factors interact with environmental and epigenetic influences, which together may amplify aggressive tendencies. While the association between genetic factors and violence is supported by some studies, conflicting evidence highlights the complexity of isolating genetic contributions to aggression. Further research is needed to understand the complex interaction between genetic, environmental, and epigenetic factors influencing violent behavior.