New evidence supports the use of twin studies to explore the effects of nature and nurture on human behaviours in childhood
By Moritz P Herle, on 5 August 2016
Written by Moritz Herle, Alison Fildes and Clare Llewellyn
Over the past century twin studies have been used to explore how nature and nurture influence individual differences in human characteristics (such as personality, intelligence or height). Identical twins share the same genes, while non-identical twins share about half of the same genes; but both types of twins grow up in the same family environment. This means that researchers can compare similarities between identical twins, and similarities between non-identical twins, to get an idea about how much differences between people in characteristics such as height are caused by nature (genes), and nurture (the environment).
The Health Behaviour Research Centre set up the Gemini twin cohort in 2007. Gemini is a landmark study of early life growth and behaviour which has been following 2400 British families with twins born in 2007. Gemini was established to help understand how genes (nature) and the environment (nurture) influence the development of eating behaviours, food preferences and growth in early childhood. Previous studies conducted by the Gemini team have suggested that individual differences in eating behaviours during childhood are strongly influenced by genes.
Like much research into early child development, these studies have had to rely on parents’ ratings of their children’s eating behaviour. This is because large sample sizes make it difficult to measure behaviours in a laboratory and because young children are unable to report accurately on their own characteristics. Parents of Gemini twins provided information about their children’s behaviour using a widely-used questionnaire called the Child Eating Behaviour Questionnaire (CEBQ). However, a criticism of twin studies is that parents might be biased by their beliefs about their twins’ zygosity (whether they are identical or non-identical) when rating each of their eating behaviours. For example, parents of identical twins might rate them more similarly simply because they think of them as ‘two peas in a pod’, while parents of non-identical twins might exaggerate the differences between them. Because twin studies are based on the comparison of similarity between identical and non-identical twin pairs, reliable and unbiased parental ratings are crucial.
We recently published a new study that set out to test if parents are biased by their twins’ zygosity when they rate their eating behaviours. Using the Gemini sample we compared eating behaviour ratings from parents who held a false belief about their twins’ zygosity (i.e. they believed them to be non-identical, when they were in fact identical) to those from parents who held an accurate belief. The only way to conclusively know whether a twin pair is identical is to conduct a genetic test, which compares the DNA of the two siblings. However these genetic tests are not routinely carried out and parents can sometimes be misinformed about their twins’ zygosity. A more thorough account of why these misunderstandings occur has been discussed in a previous study.
We established whether the Gemini twins were identical or non-identical using a combination of DNA testing and a questionnaire that accurately measures twin similarity. We also asked parents about whether they thought their twins’ were identical or not. Using this information we were able to identify parents who held a false belief about their twins’ zygosity, and those who were right. We found that approximately one third of parents of identical twins falsely believed them to be non-identical when they were about eight months old.
In order to test if parents’ ratings of their twins’ behaviours are biased by their beliefs about their zygosity, we compared the ratings of parents with false and accurate beliefs about their twins’ zygosity, on a range of eating behaviours during infancy and toddlerhood. If parent ratings were biased then we would expect identical twins whose parents believed them to be non-identical to be rated as less similar than identical twin pairs correctly identified by their parents as identical.
Interestingly, parents’ reports of their identical twins’ eating behaviours were the same, regardless of whether they had false or accurate beliefs about their twins’ zygosity. In other words, parents rated identical twins as more similarly than non-identical twins on all eating behaviours (in both infancy and toddlerhood), regardless of whether they believed them to be identical or non-identical. This indicates that parents of twins can be relied upon to provide unbiased reports of their young children’s eating behaviour, and that findings from twin studies can be trusted.
Herle, M., Fildes, A., van Jaarsveld, C., Rijsdijk, F. & Llewellyn, C. H. (2016). Parental Reports of Infant and Child Eating Behaviors are not Affected by Their Beliefs About Their Twins’ Zygosity. Behavior Genetics. doi: 10.1007/s10519-016-9798-y