Netflix has released a new documentary called You Are What You Eat, featuring identical twins who adhere to different diets over eight weeks. One twin maintains a vegan diet, while the other follows an omnivorous diet. This setup is particularly intriguing because the twins, being genetically identical, have similar health conditions before the trial begins. I won’t give away the ending for those yet to watch it, but those who prefer detailed academic studies over the excitement of a Netflix production can access the published research in JAMA Network Open.
The documentary emphasizes the significant role that twin studies play in enhancing our understanding of various subjects, extending beyond just dietary comparisons. British polymath Sir Francis Galton noted the remarkable likenesses of twins in 1875, suggesting that genetics (nature) plays a vital role in shaping our characteristics and health. Since then, twins have become a crucial element in research. So, why are twins so valuable, and how do scientists leverage them?
The Netflix documentary points out a key attribute of twins: they provide an excellent basis for comparison. Identical twins share almost all of their genes and typically grow up in the same family setting, experiencing the same parenting, education, and so on. This allows for a form of randomized controlled trial, where participants are randomly chosen for an intervention (like a new medication) and those not chosen serve as a control group (possibly receiving a placebo in drug trials). Randomized trials are generally considered the gold standard for determining the efficacy of a treatment.
However, identical twins are not necessary for conducting such trials. In fact, only a small number of trials actually include twins. Nevertheless, twins help ensure that treatment and control groups are as alike as possible, which is critical when participant numbers are limited. In the Netflix study, only 44 participants were involved. Without twins, the results would be less reliable.
For ethical or practical reasons, it’s not always possible to conduct randomized trials, and this is where twins prove useful. Consider questions like the impact of education on income or smoking on lung cancer risk. Many such questions are investigated by examining differences within pairs of identical twins (where one smokes and the other does not, for instance). By concentrating on variations between twins, genetic and shared family influences are minimized, bringing us closer to causal conclusions. While twins don’t completely eliminate the problem of missing factors, they help mitigate it.
Galton’s interest in twins originated from his curiosity about what determines who we are: our genes (nature) or our upbringing and experiences (nurture)? Although his observations were initially informal, researchers eventually established sophisticated methods to separate genetic and environmental factors.
The primary method aims to see if identical twins, who are genetically identical, are more similar than fraternal twins, who share an average of only 50% of the genes that make us unique. If identical twins show greater similarities, it suggests that genes play a significant role. Twin studies have revealed that around 40% of individual differences in personality are genetic, and genetic factors account for about 80–90% of variations in height and weight by the end of childhood.
One limitation of twin studies is their narrow focus group, meaning the findings might not be applicable to the general population. Only about 1.4% of births in Australia involve twins. Twin heritability studies also depend on robust assumptions, such as the idea that family environment impacts identical and fraternal twins equally. This issue can be somewhat addressed by studying identical twins raised apart, though such cases are rare. There have also been some misunderstandings and misuses of twin study outcomes. Galton’s emphasis on genetic factors led him to support eugenics, the notion of selective breeding for “genetic superiority,” which had harmful repercussions in the 20th century.
Nevertheless, our genes don’t dictate our fate. While certain gene combinations might increase the likelihood of traits like extraversion, who we become results from a complex interplay of genetics, upbringing, and experiences. Even genetic disorders such as Alzheimer’s are not guaranteed, and we can take steps to lower our risk. Twin studies have been conducted for decades, supported by more than 60 twin registries worldwide. In Australia, the largest such registry is Twins Research Australia, with approximately 75,000 members.