How much of our brains have been constructed before birth, and how much of it is built up over years of development? After a person is born, they absorb millions of individual memories, interactions, and events. These all contribute to fully developing and “building” a fully mature brain and behavior. However, despite the millions of experiences that each person faces every lifetime, there is always a basic genetic foundation that the brain is built upon. This is the driving question for the debate of nature versus nurture: how nature and nurture each may develop the mind and how they tie together to create the behaviors and personality of every individual.
Our inherited genetic blueprints are found in the cortical thickness inside the prefrontal cortex. Cortical thickness, the distance between gray and white matter in the brain, is the main driving force behind variation in the ‘Big Five’ personality traits. These traits include openness to experience, conscientiousness, extraversion, agreeableness, and neuroticism. Cortical thickness ranges from being around 34-80% inherited, making much of its variation highly dependent on genetics [1]. General tendency to experience negative emotions (neuroticism) and conscientiousness (self control, responsibility) is linked to higher cortical thickness. When cortical thickness is high, careful thinking and increased resistance to impulsive action is present, as well as anxiety and second guessing, connected to the heightened internal monitoring present in both conscientiousness and neuroticism. On the contrary, sociability and extraversion, agreeableness, and openness to experience tend to be linked to lower cortical thickness. These three traits are heightened in people with lower inherited cortical thickness, as a low cortical thickness decreases internal monitoring and overthinking, allowing for less chance of emotional distress and increasing a person’s chance to display more “carefree” behaviors in socializing, opening up to new experiences, and general streamlined thought and compliance. [2] However, due to cortical thickness being a polygenic trait, a person’s inherited traits and personality can vary widely, and while it plays a key role in personality inheritance, it is just one of the many different variables in brain structure that can affect inherited traits [1].
Among siblings and especially twins, genetics can play a key role in how traits are developed and expressed, even in entirely different environments. This was made especially clear in the “Jim Twins” case. In this study, two identical twins, separated at birth, ended up with similar lifestyle choices, personality traits, and even preferences in situations where their brains could have developed completely differently. From genetic inherited traits such as cortical thickness and overall frontal lobe heredity, the twins ended up extremely similar, and showed the significant impact genetics had on personality and behavior development [3].
After birth, factors such as parenting style, education, peer influence, and socioeconomic status play the role as the “nurture” for the developing brain. Each factor has its own impact on each of the Big 5 personality traits, either negative or positive. For parenting style, studies generally show an authoritative, indulgent parenting style leads to the most positive outcomes on prefrontal cortex (PFC) development, increasing conscientiousness, agreeableness, and decreasing neuroticism. On the contrary, low warmth parenting styles such as a pure authoritarian parenting style or even neglectful parenting can cause developmental issues in the PFC, heavily decreasing conscientiousness and agreeableness in neglectful parenting, and increasing sensitivity to stress in authoritative parenting [4]. With socioeconomic status, the big 5 personality traits tend to have a positive correlation. A higher socioeconomic status (SES) tends to improve openness, extroversion, and decrease neuroticism due to more exposure to diverse experiences, while low SES causes the development of neuroticism in the PFC, likely stemming from chronic stress [5]. A similar relationship is found with education, which when present can improve development in conscientiousness, openness, and emotional stability in an adolescent PFC [6].
Unlike most other external factors, peer influence doesn’t have a single, specific effect on the development of the prefrontal cortex, but can affect the PFC in different ways depending on the variables involved. Peer influence is characterized by its “spillover” effect on PFC development, often adapting conscientiousness, openness, neuroticism, and agreeableness to the same level shown by the peer group.
However, genetics can change this process. Higher genetic extroversion can actively seek out peers, speeding up the “spillover” effect on PFC development, and lower genetic neuroticism can decrease the “spillover” effect as higher emotional stability leads to less of a need to fit into a peer group [7]. These are examples of the gene-environment interaction (GxE) and gene-environment correlation (rGE) phenomenon. The rGE phenomenon occurs when an individual’s genetic makeup influences their likelihood of exposure to certain environmental conditions that may affect their behavior. For example, a person with higher extroversion is more likely to seek out peers, and therefore are more likely to be influenced by the spillover effects of peer influence. GxE, on the other hand, dictates the effect of the environment on one’s development. This is shown in people with higher genetic levels of neuroticism, which are more heavily influenced by peer influence due to lack of emotional stability and a higher need to fit in [8]. With GxE and rGE, the relationship between nature and nurture are illustrated, showing how nature and genetics can often influence either the environmental factors involved and the severity of it.
However, the opposite relationship is also found in the form of behavioral epigenetics. Behavioral epigenetics involves the effect of a person’s environment on the expression of certain behavioral genes. The change of expression in behavioral genes is mainly due to the mechanism of DNA methylation, which is the attachment of a methyl group to DNA, silencing gene expression without altering the actual genome [9]. For example, childhood trauma and stress can often cause the methylation of the NR3C1 gene, which regulates the stress response via binding cortisol. However, when turned off, cortisol cannot be binded properly, heightening stress sensitivity and increasing risks of disorders such as PTSD and depression, even when not genetically predisposed [10]. These epigenetic markers are shown to spill over to further generations, with parents passing down chemical markers to their offspring, increasing their susceptibility to mental illness [11].
The argument of nature vs. nurture is presented as a dilemma between two completely separate choices. However, both may not be as separate as many think. While each has their own primary roles in genetic predisposition in nature, and behavioral development in nurture, their jobs as a whole often blur together to become very similar. In the GxE and rGE phenomenon, nature influences the nurture of the mind, and in behavioral epigenetics, nurture can create new tendencies and even cause these tendencies to carry on in the nature of future generations. Overall, nature and nurture work in harmony, playing complex and complementary roles in developing behavior and the human mind.