History of Twin Studies

It is one of the enduring debates in psychology, stemming all the way back to our philosophical origins: What is the relationship between our biology, or nature, and that of the environment, or nurture, in shaping human behavior? In early philosophical times, this “nature versus nurture” debate was presented as an either/or phenomenon—scholars generally advocated for one side as being more influential on human behavior and outcomes than the other.

Today, however, our ability to more closely study the human body and brain in particular reveals a far messier truth: human behavior is shaped by both nature and nurture. In fact, the two are so inextricably linked that it only makes sense to separate the concepts of biology and environment for conceptual purposes. In other words, nature and nurture intersect and are virtually impossible to neatly separate, with regard to the way they impact human behavior.

Rather than asking the question of whether a given quality, personality disposition, or disease an individual develops is biologically- or environmentally-based, a more apt question is: how much of a given disposition or disorder reflects biology, and how much environment? Or, how do biology and environment interact to produce a given disposition or disease that is being studied?

How do scientists then systematically study the biological and environmental influence on human behaviors? Traditionally, twin studies have been the method of choice. Commonly referred to as the perfect natural way to experiment on humans, twin studies typically study pairs of identical, fraternal, or a combination of both twin pairs, since they already share a significant amount of genetic material.

Specifically, identical twins—also referred to as monozygotic, “mono,” denoting they develop from one fertilized egg during reproduction, then later split into two—share 100% of their genes. In contrast, fraternal twins, also referred to as dizygotic, denoting “two,” reflect two separate eggs that are fertilized by two separate sperm. Unlike identical twins, fraternal twin pairs can be opposite sex, and share 50% of their genes. In fact, fraternal twins are no more genetically similar than any other biological siblings with the same parents. However, they do share the unique feature of being paired with their sibling since conception.

Twin studies are a valuable source for studying humans because of how much of their genes are shared. In experiments where twins are recruited as participants, researchers can manipulate environmental factors to measure their effect on the pairs, while their shared genes remain the same. In such a way, nature-versus-nurture influence can be quantified and scientifically investigated. Results from such studies have far-reaching implications regarding how the body and mind respond to environmental factors.

There are a number of current pop cultural depictions of how twin studies are being used to better understand important influences on human behavior. For instance, Netflix recently released a series entitled You Are What You Eat: A Twin Experiment, which manipulates diet for an eight-week span among identical twin pairs in an effort to determine what role it plays on a number of markers of health, including weight, energy levels, sexual performance, etc.

The question of how much behaviors reflect biology versus how much what we do or develop are byproducts of the environment, or a combination of both, will never entirely be answerable. There are so many complications and unique ways that individuals respond to identical exposures in the environment, for instance. However, twin studies remain an insightful and integral methodology that enables researchers to shed light on a question that has persisted for hundreds of years.

Copyright Azadeh Aalai 2024.