Genomics Personality PDF

The Genetic Basis of Personality: A Comprehensive Overview Personality has long fascinated scholars from diverse fields, including psychology, biology, and sociology. Defined as a set of characteristic thoughts, emotions, and behaviors that make each individual unique, personality remains a central...

The Genetic Basis of Personality: A Comprehensive Overview Personality has long fascinated scholars from diverse fields, including psychology, biology, and sociology. Defined as a set of characteristic thoughts, emotions, and behaviors that make each individual unique, personality remains a central topic in understanding human behavior. While theories abound on what shapes personality, the debate has often centered around the relative contributions of nature and nurture. With the rapid advancement of genetic research, especially in the last two decades, scientists are beginning to uncover the role of genes in shaping personality. Personality traits, such as introversion, conscientiousness, and emotional stability, exhibit varying degrees of heritability, suggesting that our genetic code holds significant clues to why we behave the way we do. Yet, personality is not merely a product of genetics. Environmental factors, such as upbringing, culture, and life experiences, interact with genes to produce the complex tapestry of behaviors that define an individual. This article delves into the genetic underpinnings of personality, exploring key genes that have been linked to specific traits, the mechanisms by which genes influence behavior, and the role of gene-environment interactions. By understanding the genetics of personality, we can gain insights into not only human behavior but also the potential for personalized approaches to mental health treatment, behavioral prediction, and even education. Personality: A Complex Trait Personality is composed of multiple traits that reflect how an individual typically thinks, feels, and behaves across different situations. Some of the most widely studied traits fall under the Big Five personality dimensions: 1. Openness (creativity and openness to new experiences), 2. Conscientiousness (organization and dependability), 3. Extraversion (sociability and assertiveness), 4. Agreeableness (kindness and empathy), 5. Neuroticism (emotional instability and moodiness). These traits are influenced by both genetic and environmental factors, as demonstrated in twin and family studies. Identical twins, who share 100% of their DNA, tend to have more similar personality traits than fraternal twins, who share only 50% of their genes. However, even among identical twins, there are differences, illustrating the powerful role of the environment. While traditional theories have focused on upbringing and life experiences as primary determinants of personality, advances in molecular genetics have shifted attention to how specific genes contribute to the development of personality traits. Genome-Wide Association Studies (GWAS) and other genetic tools have revealed that many personality traits are polygenic, meaning they are influenced by many genes, each contributing a small effect.  The Role of Genetics in Personality Development Human personality is a result of complex interactions between biological, psychological, and environmental factors. Genes provide the biological blueprint that shapes the way neurotransmitters like dopamine and serotonin are regulated, influencing behaviors such as risk-taking, sociability, and emotional stability. For example, genes like DRD4 have been linked to novelty-seeking behavior, which might explain why some people are more inclined to seek out new experiences, take risks, and embrace change. Similarly, genes like SLC6A4, involved in serotonin transport, have been associated with emotional regulation and responses to stress. However, it is important to note that no single gene is responsible for a specific personality trait. Rather, it is the combined effect of many genes, along with environmental influences, that creates the unique personality profile of an individual. 1. DRD4 (Dopamine Receptor D4) – Associated with novelty-seeking behavior. 2. SLC6A4 (Serotonin Transporter Gene) – Linked to anxiety and emotional regulation. 3. COMT (Catechol-O-Methyltransferase) – Affects cognitive functions like memory and decision-making, linked to executive function and emotional control. 4. OXTR (Oxytocin Receptor Gene) – Related to social behavior, empathy, and stress response. 5. MAOA (Monoamine Oxidase A) – Connected to aggression, impulse control, and emotional regulation. 6. HTR2A (Serotonin Receptor 2A) – Influences mood, anxiety, and risk for depression. 7. BDNF (Brain-Derived Neurotrophic Factor) – Associated with learning, memory, and mood regulation. 8. TPH2 (Tryptophan Hydroxylase 2) – Involved in serotonin production, affecting mood and behavior. 9. GRIN2B – Related to cognitive function and memory formation, often linked to intelligence and problem-solving. 10. DAT1 (Dopamine Transporter) – Influences dopamine availability, linked to attention and impulsivity. Here are some additional genes involved in personality-related traits: 11. ANKK1 – Associated with dopamine receptor expression, linked to impulsivity and novelty-seeking. 12. AVPR1A – Influences social bonding and behaviors like altruism. 13. NR3C1 – Linked to stress response and resilience.  14. CACNA1C – Associated with emotional regulation and psychiatric conditions like bipolar disorder. 15. CLOCK – Linked to circadian rhythms, which can influence mood and sleep patterns. 16. FOXP2 – Involved in language and communication skills. 17. FADS2 – Associated with cognitive development and emotional well-being. 18. GABRA2 – Involved in anxiety regulation and alcohol dependence. 19. SLC6A3 – Linked to dopamine transport and implicated in ADHD. 20. GRM7 – Affects glutamate signaling, associated with mood disorders like depression. Chapter 1: Historical Background and Theoretical Frameworks 1.1 Early Theories of Personality: Humoral Theory and Phrenology The study of personality has intrigued scholars for millennia, with early attempts to explain individual differences in behavior dating back to ancient civilizations. One of the earliest frameworks for understanding personality was the humoral theory, developed by Hippocrates (circa 460–370 BCE) and expanded upon by Galen (129–216 AD). This theory proposed that human temperament was determined by the balance of four bodily fluids, or “humors”: blood, phlegm, yellow bile, and black bile. According to this view, different proportions of these humors produced various temperaments: Sanguine: Associated with an excess of blood, leading to a warm, optimistic, and social temperament. Choleric: Linked to yellow bile, causing an irritable, ambitious, and leader-like personality. Melancholic: Resulting from black bile, associated with a more introspective, serious, and often anxious demeanor. Phlegmatic: Linked to an excess of phlegm, resulting in a calm, unemotional, and slow-moving personality. Although humoral theory was grounded in ancient medical beliefs rather than empirical evidence, it laid the foundation for later attempts to link biological factors with psychological traits. Moving into the 18th and 19th centuries, phrenology emerged as another early attempt to connect biology to personality. Proposed by Franz Joseph Gall in the early 1800s, phrenology suggested that the shape of a person’s skull could reveal their personality traits and intellectual capabilities. Gall and his followers believed that different parts of the brain were responsible for various functions, and that these areas would be more developed (and thus lead to observable bumps on the skull) in individuals who excelled in certain traits. Though phrenology was eventually discredited as pseudoscience, it contributed to the growing interest in how biological factors could influence personality. 1.2 The Emergence of Behavioral Genetics In the early 20th century, as advances in biology and psychology took hold, researchers began to explore more scientifically rigorous ways to study the biological basis of personality. This period saw the emergence of behavioral genetics, a field that investigates the extent to which genetic factors contribute to individual differences in behavior and personality. One of the first approaches to studying the genetic influences on personality involved twin studies. Researchers compared the personality traits of monozygotic (identical) twins, who share nearly 100% of their genetic material, with those of dizygotic (fraternal) twins, who share approximately 50% of their genes. Early twin studies found that identical twins tended to be more similar in personality traits than fraternal twins, suggesting a genetic component to personality. These studies provided some of the first empirical evidence that personality traits were at least partially heritable. For example, the famous Minnesota Study of Twins Reared Apart, conducted by Thomas Bouchard in the 1970s and 1980s, demonstrated that identical twins raised in different environments exhibited remarkable similarities in personality, cognitive abilities, and even interests. This finding highlighted the significant role of genetics in shaping personality, while also underscoring the influence of environmental factors. Such studies contributed to the foundational understanding that both nature (genetics) and nurture (environment) play crucial roles in personality development. 1.3 Development of Personality Theories: Eysenck, Cattell, and the Big Five The mid-20th century marked a turning point in personality psychology, with the development of more sophisticated theories and models aimed at understanding the structure of personality. Two of the most influential figures in this era were Hans Eysenck and Raymond Cattell, whose work laid the groundwork for modern personality research. Hans Eysenck (1916–1997) was a pioneer in linking personality traits to biological factors, particularly the functioning of the central nervous system. Eysenck’s three-factor model of personality, also known as the PEN model, proposed that personality could be understood in terms of three broad dimensions: 1. Psychoticism: Associated with traits like aggressiveness, creativity, and impulsivity. 2. Extraversion: Reflecting sociability, assertiveness, and the tendency to seek out external stimulation. 3. Neuroticism: Characterized by emotional instability, anxiety, and moodiness. Eysenck argued that these dimensions were rooted in biological factors, such as differences in arousal levels in the brain. For example, he proposed that individuals high in extraversion had lower levels of cortical arousal, leading them to seek out stimulating environments, while introverts had higher arousal levels and preferred quieter, less stimulating settings. Raymond Cattell (1905–1998) also made significant contributions to personality research, particularly with his development of the 16 Personality Factor (16PF) model. Cattell used factor analysis, a statistical technique, to identify 16 primary personality traits, which he believed could capture the complexity of human personality. His work emphasized the importance of quantifying personality and laid the groundwork for later personality assessments. Cattell’s 16PF model was an important precursor to the widely used Big Five personality traits model, which emerged in the 1980s and 1990s. The Big Five model posits that five broad dimensions of personality capture most of the variance in individual differences: 1. Openness to Experience: Creativity, curiosity, and willingness to try new things. 2. Conscientiousness: Organization, dependability, and goal-directed behavior. 3. Extraversion: Sociability, assertiveness, and a tendency to seek stimulation. 4. Agreeableness: Compassion, cooperation, and a concern for others. 5. Neuroticism: Emotional instability, anxiety, and susceptibility to stress. The Big Five model has since become one of the most widely accepted frameworks for understanding personality and is frequently used in research exploring the genetic and environmental underpinnings of personality traits. 1.4 The Rise of Molecular Genetics in Personality Research While twin studies and personality models provided valuable insights into the heritability of personality traits, it wasn’t until the advent of molecular genetics that researchers could directly study the genetic variants associated with specific traits. With the completion of the Human Genome Project in 2003 and the development of technologies like Genome-Wide Association Studies (GWAS), researchers gained the ability to examine how variations in specific genes influence personality. One of the first genes to be linked to personality traits was the DRD4 gene, which encodes the dopamine receptor D4. Variants of this gene, particularly the 7-repeat allele, have been associated with novelty-seeking behavior and impulsivity. Research by Benjamin et al. (1996) found that individuals with this genetic variant were more likely to engage in risk-taking behaviors and seek out new experiences, providing some of the earliest evidence of a direct link between genetic variation and personality. Since then, numerous studies have identified additional genes that contribute to personality traits, including those involved in neurotransmitter systems like dopamine, serotonin, and oxytocin. These findings have advanced the field of personality genomics, offering a deeper understanding of how genetic factors shape individual differences in personality. 1.5 Moving Toward an Integrated Model of Personality As research in personality genetics has evolved, there has been a growing recognition of the importance of gene-environment interactions. While genetic factors play a significant role in shaping personality, environmental influences, such as upbringing, culture, and life experiences, also interact with genetic predispositions to influence behavior. For example, studies on epigenetics have shown that environmental factors can affect gene expression, leading to changes in behavior and personality over time. This has led to the development of more integrated models of personality that account for both genetic and environmental influences. Researchers now understand that personality traits emerge from the complex interplay between genes and the environment, making it necessary to study both in tandem to fully understand human behavior. Chapter 2: Understanding the Human Genome and Personality 2.1 Overview of the Human Genome The human genome comprises approximately 3 billion base pairs of DNA, organized into 23 pairs of chromosomes. These base pairs are made up of four nucleotides: adenine (A), thymine (T), cytosine (C), and guanine (G). The sequence of these nucleotides forms the genetic code, which provides instructions for the synthesis of proteins that regulate cellular functions, including those affecting personality. Within the genome, approximately 20,000-25,000 protein-coding genes exist. However, only a fraction of these genes directly influences personality traits. Research has identified several gene networks and biological pathways associated with personality, particularly those involved in neurotransmission, neuroplasticity, and hormonal regulation. 2.2 Genetic Variation and Its Impact on Personality Humans share about 99.9% of their genetic material, with the remaining 0.1% accounting for individual differences. This variation occurs due to mutations, single nucleotide polymorphisms (SNPs), and structural changes in DNA. These genetic differences, alongside environmental factors, shape the development of unique personality traits. Single nucleotide polymorphisms (SNPs) are the most common type of genetic variation, occurring when a single base in the DNA sequence is altered. SNPs may impact gene function or regulation and have been implicated in influencing traits such as extraversion, conscientiousness, and emotional stability. For example, a SNP in the COMT (Catechol-O-methyltransferase) gene, which affects the breakdown of dopamine in the prefrontal cortex, has been linked to variations in cognitive control and emotional resilience. 2.3 Key Genetic Pathways Linked to Personality Several biological pathways influence personality traits through the action of genes that regulate neurotransmission and hormonal signaling. Some of the key pathways include: Dopaminergic Pathway: As mentioned in Chapter 3, genes involved in the dopamine system (e.g., DRD4, DAT1) influence traits like novelty-seeking and extraversion. Dopamine is associated with the brain's reward circuitry and plays a significant role in regulating mood, motivation, and pleasure. Serotonergic Pathway: Genes related to serotonin (e.g., SLC6A4, HTR2A) impact mood regulation and are linked to personality traits such as neuroticism, emotional stability, and anxiety. Serotonin modulates emotional responses and stress sensitivity. HPA Axis and Stress Response: The hypothalamic-pituitary-adrenal (HPA) axis governs the body's response to stress by regulating the release of cortisol. Genes like NR3C1 (glucocorticoid receptor gene) affect how individuals react to stress, potentially influencing traits like emotional resilience and anxiety. 2.4 Epigenetics and Personality Epigenetics refers to changes in gene expression that do not involve alterations to the underlying DNA sequence. Epigenetic mechanisms, such as DNA methylation and histone modification, regulate when and how genes are expressed. These changes can be influenced by environmental factors, such as stress, diet, and social experiences, and may contribute to individual differences in personality. For instance, research has shown that early life stress can lead to epigenetic modifications in the SLC6A4 gene (serotonin transporter), which may increase vulnerability to anxiety and depression later in life. Similarly, changes in OXTR (oxytocin receptor) gene expression have been linked to variations in social behavior, empathy, and trust. 2.5 Gene-Environment Interactions Personality development is not solely dictated by genetics; rather, it emerges from a dynamic interplay between genes and the environment. Gene-environment interactions (GxE) occur when an individual's genetic makeup affects their sensitivity or response to environmental factors. For example, individuals with a certain variant of the 5-HTTLPR gene (involved in serotonin regulation) may be more likely to develop depression following traumatic experiences. However, those without this variant might not experience the same psychological outcomes despite similar life stressors. Gene-environment interactions highlight the importance of considering both biological predispositions and external influences when studying personality. Factors such as parenting style, education, social relationships, and even culture can modify the expression of genes linked to personality traits. 2.6 Polygenic Nature of Personality The polygenic nature of personality means that no single gene determines a person's character traits. Instead, personality results from the combined effects of many genes, each contributing a small effect. Modern techniques, such as polygenic risk scores (PRS), aggregate the effects of thousands of SNPs to estimate the genetic propensity for certain traits. For example, PRS has been used to predict the likelihood of developing psychiatric disorders like schizophrenia or bipolar disorder, which are often associated with extreme personality traits. While these scores provide insights into genetic predispositions, they do not account for the full complexity of personality development, as environmental factors also play a crucial role. 2.7 Challenges in Linking Genes to Personality Despite advances in molecular genetics, identifying specific genes that influence personality remains challenging. One reason is the complexity of gene interactions and the fact that many personality traits are influenced by a wide array of genes, each having a small effect. Additionally, environmental factors, epigenetic changes, and gene-environment interactions complicate the task of pinpointing the exact genetic underpinnings of personality. Another challenge is the replication crisis in genetic research, where findings from one study are often not replicated in subsequent studies. This inconsistency can be attributed to factors such as small sample sizes, differences in study populations, and the complexity of measuring personality traits accurately. To address these challenges, researchers increasingly rely on large-scale studies and meta-analyses, which combine data from multiple sources to achieve more reliable and generalizable results. Chapter 3: The Role of Specific Genes in Personality In this chapter, we will delve into specific genes that have been studied extensively for their role in shaping human personality. These genes are primarily associated with neurotransmitter systems and other biological pathways that influence emotional regulation, social behavior, and cognitive functioning. 3.1 Dopamine and Personality The dopamine system is integral to motivation, reward, and learning. Dopamine has been linked to behaviors such as risk-taking, impulsivity, and reward-seeking, all of which are components of personality. Several genes involved in dopamine signaling have been linked to personality traits, such as novelty-seeking, extraversion, and sensation-seeking. DRD4 (Dopamine Receptor D4 Gene) The DRD4 gene encodes a receptor for dopamine, and its polymorphisms have been linked to various personality traits. One of the most studied variants is the 7-repeat allele, which has been associated with novelty-seeking behavior, a personality trait characterized by a preference for new and exciting experiences. Role in Novelty-Seeking: Individuals with the 7-repeat variant of the DRD4 gene are more likely to exhibit high levels of novelty-seeking. They tend to engage in risk-taking behaviors, such as gambling or trying new and unconventional experiences. Studies by Cloninger et al. (1993) and Benjamin et al. (1996) have consistently demonstrated that people with this allele are more prone to exploratory behavior, which correlates with personality traits like openness to experience and extraversion. Real-World Implications: Research has suggested that the DRD4 gene may also play a role in attention-deficit/hyperactivity disorder (ADHD), as individuals with this genetic variant show higher levels of impulsivity and difficulty in focusing. DAT1 (Dopamine Transporter Gene) The DAT1 gene encodes the dopamine transporter, which is responsible for reuptake of dopamine from the synaptic cleft back into neurons. Variants of this gene, particularly the 10-repeat allele, have been linked to traits like impulsivity and attention regulation. Link to Impulsivity: The 10-repeat allele of the DAT1 gene has been associated with higher levels of impulsive behavior. Studies have demonstrated that individuals with this allele may experience challenges in impulse control, which could lead to risk-taking behaviors or substance use disorders. Research by Vandenbergh et al. (2000) has highlighted the role of this gene in contributing to impulsivity and sensation-seeking, traits that are central to certain personality dimensions, such as extraversion and openness. Association with ADHD: Like the DRD4 gene, the DAT1 gene has been implicated in ADHD. Variants of this gene are thought to contribute to dopamine dysregulation, which leads to symptoms such as inattention and hyperactivity. This suggests a genetic overlap between clinical disorders and personality traits like impulsivity and risk-taking. COMT (Catechol-O-Methyltransferase Gene) The COMT gene plays a role in breaking down dopamine in the prefrontal cortex, an area of the brain involved in executive functions, such as decision-making, planning, and emotion regulation. Variants of the COMT gene, particularly the Val158Met polymorphism, have been linked to differences in cognitive control and emotional stability. Impact on Cognitive Control: The Val variant of the COMT gene is associated with higher enzyme activity and faster dopamine breakdown, leading to lower dopamine levels in the prefrontal cortex. This has been linked to improved cognitive control but also to increased emotional rigidity and stress. On the other hand, individuals with the Met  variant tend to have slower dopamine breakdown, leading to greater emotional flexibility but potentially lower cognitive control under stress. Influence on Emotional Resilience: The Met allele has been linked to greater emotional resilience and adaptive stress responses, suggesting that individuals with this genetic variant may be better equipped to handle emotional challenges. Studies by Goldman et al. (2005) indicate that people with the Met allele demonstrate greater sensitivity to both rewards and punishment, potentially contributing to personality traits such as emotional stabilityor neuroticism. 3.2 Serotonin and Emotional Regulation The serotonin system is crucial for regulating mood, emotion, and anxiety. It is especially important in traits related to emotional stability, such as neuroticism, conscientiousness, and agreeableness. Genes involved in serotonin production, signaling, and reuptake have been linked to how individuals respond to stress and regulate their emotions. SLC6A4 (Serotonin Transporter Gene) The SLC6A4 gene encodes the serotonin transporter, responsible for the reuptake of serotonin from the synaptic cleft back into neurons. One of the most well-known polymorphisms in this gene is the 5-HTTLPR (serotonin-transporter-linked polymorphic region), which has been associated with susceptibility to stress, anxiety, and mood disorders. Short (S) vs. Long (L) Alleles: The 5-HTTLPR polymorphism comes in two main variants: the short (S) and long (L) alleles. Research has shown that individuals with the S allele are more prone to anxiety and emotional instability. They are more likely to experience depression in response to stressful life events compared to those with the L allele. Studies by Caspi et al. (2003) demonstrated a strong gene-environment interaction, where the S allele magnified the emotional impact of adverse life experiences. Impact on Neuroticism: People with the S allele also tend to score higher on personality scales measuring neuroticism, a trait characterized by heightened emotional reactivity and vulnerability to negative emotions such as anxiety, anger, and depression. This genetic predisposition suggests that the serotonin transporter gene may play a key role in shaping emotional regulation and personality development. HTR2A (Serotonin Receptor 2A Gene) The HTR2A gene encodes a receptor for serotonin and has been implicated in personality traits related to emotional reactivity and mood regulation. Variants of the HTR2A gene have been linked to disorders such as depression, schizophrenia, and bipolar disorder, all of which involve disruptions in mood and emotional stability.  Influence on Neuroticism and Emotional Reactivity: Research by Munafo et al. (2006) has shown that certain polymorphisms in the HTR2A gene are associated with higher levels of neuroticism and emotional reactivity. Individuals with these gene variants may experience greater emotional sensitivity to stress and have a higher likelihood of developing mood disorders. Role in Depression: Variants in the HTR2A gene have been found to contribute to the development of major depressive disorder (MDD). People with certain gene variants may have altered serotonin signaling, which affects their ability to regulate mood and emotions. This link between serotonin receptor functioning and mood disorders underscores the complex interplay between genetics and mental health. 3.3 Oxytocin and Social Behavior The oxytocin system is widely recognized for its role in promoting social bonding, empathy, and trust. Oxytocin, often referred to as the "love hormone," influences a wide array of social behaviors, including attachment, caregiving, and cooperation. Several genes related to oxytocin signaling have been studied for their connection to personality traits, particularly in the domain of social and prosocial behavior. OXTR (Oxytocin Receptor Gene) The OXTR gene encodes the receptor for oxytocin and plays a crucial role in social interactions and emotional regulation. Variants of this gene have been associated with individual differences in empathy, agreeableness, and stress reactivity. Role in Empathy and Trust: Studies by Rodrigues et al. (2009) have shown that certain variants of the OXTR gene are linked to higher levels of empathy and social trust. Individuals with these variants tend to be more attuned to the emotional states of others and are more likely to engage in prosocial behaviors, such as helping or comforting others. Impact on Stress Response: Variants in the OXTR gene have also been associated with differences in how individuals respond to stress. People with certain OXTR variants may exhibit lower cortisol levels in response to stress, suggesting that oxytocin influences both social bonding and stress resilience. This connection between oxytocin signaling and stress regulation highlights its role in fostering emotional stability and reducing anxiety in social contexts. AVPR1A (Vasopressin Receptor Gene) The AVPR1A gene encodes a receptor for vasopressin, a hormone closely related to oxytocin. Like oxytocin, vasopressin plays a role in regulating social behaviors, particularly those involving dominance, territoriality, and cooperation.  Association with Altruism: Research by Knafo et al. (2008) found that certain variants of the AVPR1A gene were linked to altruistic behavior. People with these gene variants were more likely to engage in selfless acts, such as donating to charity or helping others without expecting personal gain. Influence on Social Bonding: In addition to its role in altruism, the AVPR1A gene has been associated with social bonding and the formation of close interpersonal relationships. Variants of this gene may affect how individuals form attachments to others, particularly in romantic and familial relationships. 3.4 Genetic Interactions and Personality One important aspect of personality genetics is how multiple genes interact to influence behavior. For instance, an individual may inherit genetic variants that make them more sensitive to both rewards (dopamine system) and social cues (oxytocin system), leading to a unique combination of traits like extraversion and agreeableness. Moreover, the interaction between serotonin and dopamine pathways has been studied for its role in emotion regulationand impulsivity. Individuals with certain variants in both pathways may be more prone to mood swings, impulsivity, or risk-taking behavior. References: 1. Cloninger, C. R., Svrakic, D. M., & Przybeck, T. R. (1993). A psychobiological model of temperament and character. Archives of General Psychiatry, 50(12), 975-990. 2. Benjamin, J., Li, L., Patterson, C., Greenberg, B. D., Murphy, D. L., & Hamer, D. H. (1996). Population and familial association between the D4 dopamine receptor gene and measures of Novelty Seeking. Nature Genetics, 12(1), 81–84. 3. Caspi, A., Sugden, K., Moffitt, T. E., Taylor, A., Craig, I. W., Harrington, H.,... & Poulton, R. (2003). Influence of life stress on depression: moderation by a polymorphism in the 5-HTT gene. Science, 301(5631), 386-389. 4. Vandenbergh, D. J., Persico, A. M., Hawkins, A. L., Griffin, C. A., Li, X., Jabs, E. W., & Uhl, G. R. (2000). Human dopamine transporter gene (DAT1) maps to chromosome 5p15.3 and displays a VNTR. Genomics, 14(4), 1104-1110. 5. Knafo, A., Israel, S., Ebstein, R. P., & Gvirtz, S. (2008). Individual differences in altruism: The role of the vasopressin receptor gene AVPR1A and testosterone. Social Cognitive and Affective Neuroscience, 3(4), 294-301. 6. Tost, H., Kolachana, B., Hakimi, S., Lemaitre, H., Verchinski, B. A., Mattay, V. S.,... & Weinberger, D. R. (2010). A common allele in the oxytocin receptor gene (OXTR) impacts prosocial temperament and human hypothalamic-limbic structure and function. Proceedings of the National Academy of Sciences, 107(31), 13936-13941.