Nature or nurture The twin study

Nature refers to all of the genes and hereditary factors that influence who we are—from our physical appearance to our personality characteristics.

Nurture refers to all the environmental variables that impact who we are, including our early childhood experiences, how we were raised, our social relationships, and our surrounding culture.

The relative importance of nature and nurture to how a child develops has been debated by philosophers and psychologists for centuries, and has had strong — and sometimes misguided — influences on public policy.

Children learn from what they see around them, and if what they mainly experience is violence, abuse, truancy and no expectations for success, their chances for a wholesome future are compromised from the start. As my son Erik Engquist, a fellow journalist who was Twin A, put it: “Genes define your potential, but your environment largely determines how you turn out. The few who escape negative influences are outliers.”

Some philosophers such as Plato and Descartes suggested that certain things are inborn, or that they occur naturally regardless of environmental influences. Nativists take the position that all or most behaviours and characteristics are the results of inheritance.

Other well-known thinkers such as John Locke believed in what is known as tabula rasa, which suggests that the mind begins as a blank slate. According to this notion, everything that we are and all of our knowledge is determined by our experience.

Even today, research in psychology often tends to emphasize one influence over the other. In biopsychology, for example, researchers conduct studies exploring how neurotransmitters influence behaviour, which emphasizes the nature side of the debate. In social psychology, researchers might conduct studies looking at how things such as peer pressure and social media influence behaviours, stressing the importance of nurture.

In one of the purest twins experiments ever designed, Mason was part of the team that compared the effects of a year spent in space on 52-year-old astronaut Scott Kelly with the Earth-based experience of his identical twin, Mark. Researchers discovered that Scott Kelly’s time on the International Space Station had altered the expression of 7 percent of his genes, including those involved in blood oxygenation and DNA repair. “It was surprising how many genes responded to the stress of spaceflight,” Mason says.

Mason says the research offers insight into how to study the effects of harsh environments. “We want to leverage the capacity of twin studies to understand human physiology at extremes, such as scuba diving, climbing Mount Everest or flying fighter jets,” he says.

The intense interest in how genes affect our lives has inspired scientists around the world — including in the United States, the Netherlands, Denmark, China and Cuba — to create large national registries of twins. The largest of these, which has data on 85,000 pairs of Swedes, is being used to research allergies, cancer, dementia, cardiovascular disease and other topics.

One of the goals of the Older Australian Twins Study is to discover the genetic underpinnings of cognitive decline as people age. Using brain scans of 92 pairs of identical twins and other tests, Sachdev concluded that 70 percent of cerebral small vessel disease, a type of brain lesion associated with dementia, is genetically determined. He hopes that mapping the twins’ whole genome and then comparing their epigenomes to see which genes were turned on will reveal the mechanisms behind the development. “The ultimate goal would be a drug that would prevent or stop the disease from progressing,” he says.

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Twins studies have been so popular that one 2015 meta-analysis found that researchers had looked at no fewer than 17,800 traits — including depression, cardiovascular disease and gun ownership — involving more than 14.5 million twin pairs over the past 50 years. It concluded that both “nature” (what you’re born with) and “nurture” (what you’ve been exposed to as you age) are nearly equally important for understanding people’s personalities and health: The variation for traits and diseases was, on average, 49 percent attributable to genes and 51 percent to environment.

References:
(1) The Age Old Debate of Nature vs. Nurture by Kendra Cherry
(2) Scientists see twins as the perfect laboratory to examine the impact of nature vs. nurture by Sarah Elizabeth Richards
(3) What Twins Can Teach Us About Nature vs. Nurture by Jane E. Brody
(4) What makes us? Nature or nurture? The DNA debate comes back to life by Robin McKie