Still from You Are What You Eat. Netflix

A new Netflix documentary, You Are What You Eat, showcases sets of identical twins as they adopt different diets. For eight weeks one twin follows a vegan diet while the other one follows an omnivorous diet. The experiment is compelling because, being genetically identical, the health of each twin is very similar before the trial.

I won’t spoil the ending for those who haven’t seen it, but if you prefer the drab writings of academics over the glitz and glamour of Netflix, you can read the published paper in the journal JAMA Network Open.

The documentary underscores the extraordinary contribution twin studies make to advancing our understanding of the world. But this goes well beyond comparing different diets.

British polymath Sir Francis Galton first documented the striking similarities of twins in 1875, arguing this proved “nature” was an important contributor to our dispositions and health. Since then, twins have been used extensively in research. What is it then that makes twins so special, and how do researchers harness the power of twins?

Twins as comparisons for each other​

The Netflix documentary highlights one important feature of twins – they are good for making comparisons. Identical twins share nearly all of their genes, and usually grow up in the same household, meaning they experience the same parenting, schooling and so on.

The documentary is an example of a randomised controlled trial, where participants are selected at random for some intervention (like a new drug) and those not selected serve as controls (in drug trials they might get a placebo). Randomised trials are normally seen as the gold standard in evaluating what works.

We don’t actually need identical twins for such trials. In fact, relatively few trials use twins. But twins can help to ensure the treatment and control groups are as similar as possible. This is especially important when there are few participants. In the Netflix study, there were only 44. Without twins, their results would have been more uncertain.

For ethical reasons or just sheer practicality, we can’t always run randomised trials. This is where twins can help us out. Say we want to know the effect of education on earnings, or the effect of smoking on developing lung cancer.

These questions, and many more, have been tackled by comparing the differences within sets of identical twins (for example, where one twin smokes and the other doesn’t).

By focusing on differences between twins, we eliminate genetic and common family factors, and can be more confident about causality. Using twins doesn’t fully solve the problem of omitted factors, but it helps to reduce it.

Still image from You Are What You Eat. Netflix​

Twins and heritability​

Galton’s fascination with twins stemmed from wanting to know why we are the way we are. Is it our genes (nature), or is it our upbringing and personal experiences (nurture)? His early observations were informal, but eventually researchers developed sophisticated methods to disentangle genes and environment.

The main approach is essentially to see whether identical twins, who are genetically the same, are more similar than fraternal twins, who on average share only 50% of the gene variants that make us unique. If the identical twins are more similar, that indicates genes matter.

For example, twin studies have shown that around 40% of individual differences in personality can be explained by genes, along with as much as 80–90% of differences in height and weight at the end of childhood.

Limitations and misuse of twin research​

One drawback to twin studies is that twins are a select group, and findings may not always generalise to the broader population. Only about 1.4% of births in Australia are twins.

Twin heritability studies also rely on strong assumptions, like that the common family environment matters to the same extent for identical and fraternal twins. This can be overcome by focusing on identical twins reared apart, but these studies are rare.

There has also been some misunderstanding and misuse of findings from twin studies. Galton’s belief in the power of nature led him to promote eugenics, the idea of selective breeding to achieve “genetic superiority”, which had devastating consequences in the 20th century.

But our genes are not our destiny. While a certain combination of genes may raise the likelihood you’re extraverted, who we grow up to be is a complicated interplay between genes, upbringing and personal experiences. Even disorders with a genetic basis, like Alzheimer’s, are rarely certain to occur, and we can take actions to reduce our risk.

Identical twins are actually quite rare. Shutterstock​

The future of twin research​

Twin studies have been conducted for decades, with more than 60 twin registries operating around the world. In Australia, the largest registry is Twins Research Australia, who have around 75,000 members.

With more opportunities to link data, genome mapping, and advances in machine learning, what we can learn from twins will only increase in the future. And we can expect twins to continue to play a key role in advancing our understanding of the world for many years to come.

This article was first published on The Conversation, and was written by , Nathan Kettlewell, Senior lecturer, Economics Department, University of Technology Sydney

 

[Penny4]

I saw that show. The male twins on that top photo were popular on the programme, due to their looks.

 

[Cyberchook]

I saw that show when it was released and recommend it to anyone who eats....the western diet is severely corrupted from pasture to mouth, so no wonder people have health issues all the time. It would.be very much the same here in i. In Australia, the farming methods I was involved with 50-plus years ago, have long since gone. Crowded hospitals and long lists for surgeries will increase as I do not see any common sense being used by those who place profit first no matter how it is achieved, with offering additive 'food' maybe a lettuce leaf or two and disregarding the damage and cruelty to animals to achieve this is heartbreaking in their products to the populations. The twin experiment is great and the vegetarians showed up the value of healthy food and not only weight, but energy and general health. It was not a long-time experiment, but it shows just where our kids are heading..

 

[AlanQ]

Watched this show, far too much finger wagging at meat eaters. Clearly was a bias in the production, most likely that bias would be more difficult to find in the actual publish paper that will also have peer reviews on the findings. For those that get their knowledge from a TV production you need to know the Da Vinci Code was fiction.

 

[Penny4]

Watched this show, far too much finger wagging at meat eaters. Clearly was a bias in the production, most likely that bias would be more difficult to find in the actual publish paper that will also have peer reviews on the findings. For those that get their knowledge from a TV production you need to know the Da Vinci Code was fiction.

I thought it was biased too. Dr. Gardner from Stanford who conducted the study personally follows a plant based diet.

 

[siameezer]

It is just vegan propaganda and not at all based on scientific facts. The facts have been twisted to meet the agenda of the plant-based fans, and most of the claims can be easily debunked. Fortunately, I am not easily swayed by propaganda.