Who Are We, Really? Understanding Genetics, Environment, and Evolution

Reading time: 6 min

Have you ever wondered how much of who you are was already determined when you were born, and how much was shaped by your choices and experiences? To what extent do life experiences influence personality, and how much of our behavior is rooted in biology? These questions have fascinated psychologists for decades and gave rise to the well-known debate between nature and nurture.

Nativists, representing the “nature” side, believe that genetics plays the main role in determining psychological characteristics, whereas empiricists, supporting the “nurture” perspective, argue that environmental and social influences are more important. But who is actually correct?

A specific branch of psychology, called behavioral genetics, attempts to answer this question by studying the role of genes in shaping human behavior and psychological traits. Behavioral genetics focuses on understanding the contribution of genetics to characteristics such as personality, intelligence, temperament, psychological disorders, and individual differences. However, rather than looking for a single “gene” responsible for behavior, this field investigates the interaction between genetic predispositions and environmental influences.

To study these factors, behavioral geneticists mainly rely on two methods: studies on identical twins and studies on adopted siblings. Identical twins share nearly the exact same genetic makeup, making them particularly useful for understanding the role of environmental influences on individual differences. If identical twins raised in the same household still show differences in personality or behavior, these variations are likely influenced by environmental factors. On the other hand, studies on adopted siblings help researchers understand the impact of the environment. Since adopted siblings do not share biological ties, any similarities between them are more likely to result from being raised in the same family context.

An especially important contribution to behavioral genetics comes from studies on identical twins separated at birth. Even though these twins grow up in different households and environments, researchers have often found similarities in personality traits, preferences, and behavioral tendencies.

One famous example is the case of Jim Lewis and Jim Springer, identical twins who were separated at birth and reunited only at the age of 39. Despite never having met before, they discovered several striking similarities: both had a dog named Toy, both worked in similar professional fields, and both suffered from severe headaches. Cases like this have often been used to suggest a genetic contribution to behavior, although they should be interpreted carefully and not as proof of complete genetic determinism.

Another important concept in behavioral genetics is heritability, defined as the proportion of variability in a trait attributable to genetic differences within a population. For example, height is considered highly heritable, meaning that genetic differences explain a large proportion of height variation within a population. However, this does not mean that a person’s height is “90% genetic” as an individual. Rather, it means that within a specific population, most differences in height are explained by genetic variation, while environmental factors such as nutrition still play an important role.

Research in behavioral genetics has also shown that personality traits are influenced by both genes and the environment. Studies suggest that approximately 40–50% of personality variation may be explained by genetic factors, while the remaining proportion is influenced by environmental experiences.

Personality psychologists usually distinguish between shared and non-shared environments. Shared environment refers to experiences that siblings have in common because they grow up in the same household, such as family values, socioeconomic conditions, religion, nutrition, or the educational resources available at home. Non-shared environment, instead, refers to the unique experiences that make siblings different from one another despite growing up in the same family. These may include friendships, personal interests, different life experiences, and even the distinct relationship each sibling develops with their parents.

For this reason, siblings can grow up in the same house, share part of the same genetic makeup, and still develop very different personalities. Behavioral genetics suggests that who we become is not determined exclusively by either biology or environment, but rather by the continuous interaction between both.

Behavioral genetics is not only interested in personality traits, but also in biological individual differences, such as circadian rhythms. Are you an early bird who prefers to wake up early and feels more productive during the first hours of the day, or are you a night owl who prefers staying up late and functions better in the evening? Both tendencies are completely normal and appear to be partly influenced by genetics.

Circadian rhythms regulate several biological processes, including hormone secretion, body temperature, and the sleep–wake cycle. For most people, this internal cycle lasts approximately 24 hours. However, individual differences exist: some people may have slightly shorter circadian rhythms, leading them to wake up earlier and feel more energetic in the morning, whereas others may have slightly longer rhythms, making them naturally inclined to stay awake later and wake up later in the day.

A related perspective that further emphasizes the role of biology in human behavior is evolutionary psychology. While behavioral genetics focuses on measuring the contribution of genes to psychological traits, evolutionary psychology attempts to explain why certain behaviors and psychological tendencies may have developed over time.

According to evolutionary psychologists, many human behaviors may be the result of adaptations that helped our ancestors survive and reproduce. These adaptations are often referred to as evolved psychological mechanisms, meaning psychological tendencies that developed through natural selection because they increased the chances of survival.

An example of this can be found in food preferences. From an evolutionary perspective, our ancestors developed a preference for calorie-dense and sweet foods because they were generally safer to consume than bitter substances, which in nature are more often associated with toxins or poisonous plants.

In conclusion, understanding human behavior requires considering both biological and environmental influences rather than focusing exclusively on one perspective. Behavioral genetics has shown that traits such as personality, intelligence, and even circadian rhythms are partly influenced by genetics, while environmental experiences continue to shape important individual differences. At the same time, evolutionary psychology offers an explanation for why some behavioral tendencies may have developed, suggesting that certain psychological mechanisms evolved because they improved survival and reproduction. Therefore, the debate between nature and nurture should not be understood as a competition between opposites, but rather as an interaction in which genes, experiences, and evolutionary influences all contribute to shaping who we are.

By Benedetta Anselmi

benedetta.anselmi01@icatt.it

References

Psicologia della personalità, cured by Marina Giampiero, Paola Iannello, Charles S. Carver and Michel F. Scheier

Article “Separati alla nascita: la straordinaria storia vera di gemelli identici estraniati che hanno vissuto la stessa vita.” on the online review “Flonchi online media”