How Do Genes and Environment Influence Human Development? An Empirical Study Investigating Twins

For over 20 years, I have been researching to scientifically clarify how genes and environment affect humans. My research method is the twin study. Although I have long studied adult twins, in December 2004, I launched the "Tokyo Twin Cohort Project" to examine the influence of genes and environment on over a thousand pairs of infant twins.

Identical twins are genetically identical and share the same upbringing environment. Fraternal twins also share the same upbringing environment but are only 50% genetically similar. We periodically conduct surveys on their intelligence, language ability, personality, and other traits, and then compare the data. If the results for identical and fraternal twins are similar, it suggests a strong genetic influence, whereas differences indicate a strong environmental influence.

This study has revealed several interesting findings. For instance, even with the same genetic predisposition, its expression can vary depending on the environment. The genetic predisposition for behavioral problems tends to be more pronounced in families with overly strict or inconsistent discipline, while it is less likely to manifest in families with consistent and appropriate discipline, even if the genetic predisposition exists. Furthermore, our results show that children who are read to develop higher problem-solving skills, and those who are given the freedom to explore without being forced into activities tend to develop higher intellectual abilities.

While many of the survey's findings were, of course, as expected, we were able to substantiate previously asserted claims with data. What is fascinating is that the influence of genes and environment changes dynamically depending on the circumstances. For example, when tracking weight, newborns are overwhelmingly affected by the uterine environment, but as they grow, the influence of genetics becomes more pronounced. Similarly, the impacts of genes and environment on intellectual abilities and personality also change dynamically.

I am often asked which is more important: genes or environment. But both are crucial and cannot be considered in isolation. Genes and environment are intricately intertwined, each playing a vital role. Life phenomena are inherently complex. It is important to understand them in all their complexity, rather than trying to reduce them to simplistic theories.

The vast survey results from the "Tokyo Twin Cohort Project" form a valuable database that will aid various future research projects. In the future, I hope to conduct research that links these findings to educational practices and policies.