Scientists have studied twins for many years to understand how genes and environments influence differences among individuals, spanning conditions such as cancer and mental health to characteristics such as intelligence and political beliefs.
Although the twin method is well-established, findings from twin studies are often controversial. Critics of twin research question the value of establishing that characteristics, such as health behaviours, have a strong genetic basis. A primary concern is that these types of findings will result in complacency or fatalism, effectively undermining motivation to change lifestyle. But there is very little evidence to support these fears.
Genetic influence on human characteristics is often misinterpreted. It is wrongly assumed that a behaviour that has strong genetic influence (highly heritable) must be biologically hardwired. However, genes are not destiny. Genes are often dependent on environmental exposure, such that genes can have a stronger effect, or no effect, depending on the environment.
For example, people with a genetic predisposition to lung cancer are unlikely to develop the disease unless they smoke. The same is true of behaviour. Behaviour is only elicited in response to environmental cues. Establishing that a behaviour has an important genetic basis does not imply that this behaviour cannot be changed through environmental means.
What’s the benefit?
Twin studies provide important insights into when and how genes and environments shape human nature. Studies following twins over many years have shown that the importance of genes can change dramatically with development . Genetic influence tends to increase with age for many characteristics – for example, body weight and intelligence. It is thought that with increasing maturity comes the ability to make independent choices in line with our genetic predispositions. For example, a child who is genetically predisposed to be good at reading might join a library to gain access to more books and meet like-minded people once he or she is a teenager. Twins can therefore identify the windows of opportunity when environmental influences might be strongest, and when behaviours may be easier to change.
Twin studies also inform researchers where best to target environmental interventions. Interventions targeting characteristics influenced by shared environments might best be directed at the family environment. But policymakers may have greater success if interventions are oriented towards the wider environment for characteristics shaped by factors unique to each person.
On a broader level, twin studies are also the first step towards molecular genetic research identifying specific genes involved. One classic example is body weight. We have known from twin studies that weight has a strong genetic basis, which led researchers to identify approximately 100 genetic variants involved.
The most important of these is FTO (the fat mass and obesity gene); and adults carrying two copies of the risk variant are heavier and at increased risk for obesity. The discovery of FTO and other variants paved the way for researchers to study the mechanisms through which genes influence weight in order to develop new drugs, and to help people with obesity to understand their vulnerability better.
What are the risks?
Undeniably there are concerns that promoting the knowledge that healthy behaviour is partially down to genes may somehow stop people from taking responsibility for managing their own, or their child’s behaviour. However, studies exploring individual feedback on DNA-based disease risk suggest that knowing your genetic predisposition does not necessarily undermine attempts to improve health, but may increase engagement and motivation to change behaviour.
Evidence that children’s behaviours are partially influenced by their genes also serves to alleviate the blame that often rests on parents. For example, our recent study establishing considerable genetic influence on toddlers’ fussy eating could help to ease the guilt and frustration parents experience when dealing with an extremely fussy child.
Twin research has undoubtedly advanced our understanding of human nature and has revolutionised the way we discuss the complicated relationship between nature (genes) and nurture (environment). Twin research has also led to breakthroughs in molecular genetic research that has the potential to change the course of disease treatment. Twins remain a valuable tool for researchers to establish the lay of the land in relation to the complexity of human nature.
Andrea Smith, PhD candidate in Epidemiology/Public Health, Health Behaviour Research Center, UCL; Alison Fildes, University Academic Fellow, University of Leeds; Clare Llewellyn, Lecturer in Behavioural Obesity Research, UCL, and Moritz Herle, PhD candidate in Epidemiology/Public Health, Health Behaviour Research Center, UCL