Brains are incredibly complex. The adult brain consists of about 86 billion neurons constantly at work, consuming 20% of the body's energy. The prenatal period and early childhood are when the brain develops most rapidly, with newborns and toddlers forming up to 1000 connections per second. But our brains continue to change and rewire throughout our lifespan.
In this article, we'll discuss the environmental and genetic factors that contribute to healthy brain development throughout the lifespan. How and do we know if one factor is more important, this is an ongoing debate amongst psychologists called the nature and nurture in development debate.
Our brain develops throughout our life, freepik.com.
Role of nature and nurture in human development
Our brain development is intrinsically connected to our cognitive, psychological or social development. But what factors shape this process? One way to look at this question is in light of the nature-nurture debate.
The nature-nurture debate of brain development concerns whether our neural structures developed is a product of our genes or our environment determines how the brain develops.
Nature and nurture factors in human development
When considering the influence of nature, we focus on the genes we inherit from our biological parents.
Some examples of nurture factors are the physical environment we grow up in, nutrition, social interactions with the caregiver or peers, how intellectually stimulated we are or the social and cultural norms we learn.
In the area of brain development, we see evidence that genetic and environmental influences play an important role, consistent with the interactionist approach.
The interactionist approach proposes that our development is influenced by an interaction of our genetic predispositions and environmental factors.
Influence of nature and nurture in child development
It might not be easy to separate nature from nurture's influence on child development. How do we know if the way a child develops is because they inherited a specific set of genes or due to environmental factors such as parenting?
Difference between nature and nurture in child development
One way to investigate nature influences independent of nurturing factors is by studying newborns that haven't had the time to be influenced by environmental factors. Keep in mind that not all genetically influenced traits will be developed at the time of birth, which is an important limitation of this method.
Meltzoff and Moore (1977) investigated whether the ability to imitate the facial expressions of others is innate. They found that babies 12-21 days old imitated the facial expressions shown to them. However, these findings were later refuted by more rigorous studies (Oostenbroek et al., 2016).
It's also important to remember that even newborns have already been exposed to environmental influences in the womb. We know that certain factors, like the mother's diet or alcohol consumption, can result in changes to the fetus's brain development.
Difference between nature and nurture in child development research
To quantify the extent to which genetic and environmental influences particular aspects of development, we can use family studies like adoption studies or twin studies.
Twin studies use pairs of monozygotic (MZ) twins and dizygotic (DZ) twins to estimate the influence of shared genes, as well as shared and unshared environments, on their development.
Both MZ and DZ twins are likely to share a large portion of their developmental environment, while only MZ twins share 100% of their genetic material (DZ twins share 50%).
Twin studies calculate the heritability of a trait by comparing the concordance rates between MZ and DZ twin pairs. Concordance rates show the probability that both twins develop the same trait. If genes influence a trait, we would expect the concordance rates between MZ twins to be significantly higher than between DZ twins.
A large-scale twin study in the UK (TEDS) found that genetics play a huge role in language and cognitive development, behaviour problems, academic achievement and typical and atypical development (Touton, Spinath & Plomin, 2002).
If a child develops traits consistent with their biological parents, it suggests that the feature is likely to be inherited. On the other hand, if the children develop characteristics dissimilar to birth parents or similar to adoptive parents, it suggests that it was their environment that influenced those traits. These are tested using adoption studies.
Monozygotic twins can grow up to become remarkably similar, but questions are raised about the role of nature and nurture factors, freepik.com.
Nature and nurture in intellectual development
Research has found evidence of both nature and nurture factors influencing intellectual development.
The Minnesota Study of Twins Reared Apart (Bouchard et al., 1979) investigated the influence of nature on intellectual and personality development using the twin study design. The sample was recruited from several countries worldwide and included over a hundred twin pairs separated at birth and who grew up in different families. At the time of the study, participants were, on average, around 30 years old.
The researchers measured intelligence using a range of IQ tests.
The study claimed that 70% of the variance in intelligence is due to genes only. These results strongly support the role of nature in brain development. The authors famously concluded that MZ twins that were raised apart are as similar as MZ twins that grew up together, almost discrediting the environmental influence on human development.
To highlight the similarity between twins raised together, the authors mention cases such as the 'Jim twins'. Despite the twins being separated shortly after birth, both got named James by their adopted families and had a brother named Larry. In adulthood, they got romantically involved with women sharing the same name; they chose the same occupation, trained in the same area and regularly visited the same holiday destinations.
The Minnesota twin study has been widely criticised for methodological issues and bias. Later, twin studies suggest that the heritability of intelligence is closer to 50%.
The results of the Minnesota twin study have been later refuted, as the authors omitted a critical comparison from their study – the comparison between MZ and DZ twins.
As this comparison data surfaced, it was found that there was no difference between the groups, suggesting that there, in fact, may be no genetic influence on intelligence.
So, where did this similarity found in the study come from? It turns out that most of the twins “reared apart” in the study were actually not separated at birth, often lived with a family member and even maintained contact throughout the years (Joseph, 2022).
The animal study of Rosenzweig et al. (1972) supported the influence of nurture on brain development. The study investigated whether the environment in which rats developed influenced their brain's biology.
In this experiment, a pair of rats from the same litter was separated; one rat was placed in a stimulating environment while the other in an impoverished environment.
The stimulating environment (SE) included the company of several more rats in an environment where there were various objects to interact with.
The rats, placed in an impoverished environment (IE), lived in a smaller cage alone and without any objects to play with.
After a few weeks, the brains of the rats were studied post-mortally. The researchers found a significantly larger volume and thickness in the cerebral cortex of SE rats. The cerebral cortex plays many important cognitive functions like vision, memory etc. The SE rats didn't produce more neurons; instead, they produced larger neurons and synapses. The study indicates that our environment plays a significant role in development.
Nature and nurture in language development
While none of us is born fluent in a language, some psychologists argue that the ability to adopt a language's complex rules quickly is innate; this approach to language acquisition is known as nativism. According to Noam Chomsky's theory of universal grammar, children are born with the innate knowledge of grammatical classes. And as children become more exposed to their native language, they automatically deduce and apply grammatical rules.
When a child hears “Mum drinks water”, they automatically learn the subject verb object grammatical order and can generalise it to other sentences. So, when the child learns the word “juice”, they will be able to say “I want juice” instead of “I juice want”.
On the other hand, the constructivist approach to language acquisition proposes that language is learned rather than innate. Lev Vygotsky, who developed the sociocultural theory of cognitive development, argued that language develops through social interactions with more knowledgeable others, most often their caregivers. This theory argues that language is a key cultural tool that guides children through cognitive development. With age, language also becomes internalised, forming the child's inner speech.
Language development is influenced both by nature and nurture, freepik.com.
Importance of nature and nurture in personality development
Another aspect related to brain development is personality.
Personality refers to our stable patterns of thought, affective state and behaviour that we maintain across various situations. The Big Five model identifies five main personality traits: extraversion, neuroticism (or emotional stability), conscientiousness, agreeableness, and openness to experience.
Some research suggests that personality results from an interaction between genetic and environmental influences.
Jang and colleagues (1996) investigated the heritability of the Big Five personality traits. Their study compared shared personality traits in 123 pairs of MZ twins and 127 pairs of DZ twins. The researchers found that each of the five personality dimensions had a heritability estimate ranging from 41% to 61%, suggesting that genes account for around half of the variation in personality.
If genes solely shape personality, then concordance rates would be 100%. As this is not the case, other factors must influence our personality.
Even though personality is generally stable, we see changes in different personality traits throughout the lifespan; this suggests that nurture must have some form of role in our personality development.
As we become older and more mature, we tend to become more emotionally stable, agreeable and conscientious. A rapid increase in these traits is seen particularly in young adulthood, but aspects of personality can change at any moment in our life.
The dynamic nature of personality can be attributed to our changing social and occupational roles or important life events. These changes will vary for everyone; they can occur at different paces and extents. So what is it that causes these differences? Our environment or our genes?
Nature and Nurture in Development - Key takeaways
- The nature-nurture debate of brain development concerns whether our neural structures develop as a product of our genes or whether our environment determines this.
- Human development and the role of nature versus nurture can be investigated through family studies (adoption and twin studies), animal studies or studies on newborns.
- We see that both environmental and intellectual factors play important roles in intellectual development. Moreover, the environment can cause biological changes in the size of neurons.
- We also see both types of influences in language development, with nativistic theories stressing the influence of nature on language acquisition and constructivist theories highlighting the role of nurture.
- Genes likely account for around half of the variation in personality variability. However, the influence of nurture cannot be dismissed, which is apparent as personality tends to change throughout the lifespan.
References
- Bouchard, T. J., Lykken, D. T., McGue, M., Segal, N. L., & Tellegen, A. (1990). Sources of human psychological differences: The Minnesota Study of Twins Reared Apart. Science.
- Joseph, J. (2022). A reevaluation of the 1990 “Minnesota Study of Twins Reared Apart” IQ study. Human Development.
- Meltzoff, A. N., & Moore, M. K. (1977). Imitation of facial and manual gestures by human neonates. Science (New York, N.Y.).
- Rosenzweig, M. R., Bennett, E. L., & Diamond, M. C. (1972). Brain changes in response to experience. Scientific American.
- Tellegen, A., Lykken, D. T., Bouchard, T. J., Jr, Wilcox, K. J., Segal, N. L., & Rich, S. (1988). Personality similarity in twins reared apart and together. Journal of Personality and Social Psychology.
- Jang, K. L., Livesley, W. J., & Vernon, P. A. (1996). Heritability of the big five personality dimensions and their facets: a twin study. Journal of Personality.
- Bleidorn, W., Schwaba, T., Zheng, A., Hopwood, C. J., Sosa, S. S., Roberts, B. W., & Briley, D. A. (2022). Personality stability and change: A meta-analysis of longitudinal studies. Psychological Bulletin.
- Oostenbroek, J., Suddendorf, T., Nielsen, M., Redshaw, J., Kennedy-Costantini, S., Davis, J., Davis, J., Clark, S. and Slaughter, V. (2016). Comprehensive longitudinal study challenges the existence of neonatal imitation in humans. Current Biology.
- Trouton, A., Spinath, F. M., & Plomin, R. (2002). Twins Early Development Study (TEDS): A Multivariate, Longitudinal Genetic Investigation of Language, Cognition and Behavior Problems in Childhood. Twin Research and Human Genetics.
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