Is it our genome that determines who we are, or is it our environment? This question has occupied science more since Darwin’s time. Early geneticists saw humans as puppets on a string of their genes. Psychologists and sociologists objected to this and insisted education and experience had an influence. Meanwhile, it has become evident that it is neither size, weight, intelligence or interpersonal communication that determines who we are. The fight continues, however, with respect to their relative contributions – until now.
In the largest twin study to date, scientists have evaluated data from 14.5 million twin pairs from 39 countries (1). Researchers, under the guidance of statistical geneticist Danielle Posthuma, have looked at 2,748 twin studies, all published between1958 and 2012. A total of 17,000 human traits were screened in these studies, not only of biological but also psychological nature.
Their summary: Differences between humans are inherited by 50% on average. Traits can never be explained by external factors alone, even though the relative contribution of the environment and genes may vary substantially between single characteristics. In fact, the scientists of the Vrije-Universität in Amsterdam did not identify a single feature that did not depend on any genetic influence in their meta-analysis. Even the degree to which study participants were devoutly engaged seemed to involve heritage. It was long believed a that a poor education may be responsible for unsocial behaviors in children; studies show, however, that genes may play at least a minor role.
Even smoking habits are partially influenced by genes. But this does not allow to blame genes for all unhealthy behaviors, even if such a trait would be inherited by 100%.
The meta-analysis shows that in about 2/3 of all investigated traits, the similarity between monozygotic twins (which share 100% of their genes) exactly double that of dizygotic twins (that share 50% of genes but 100% of an environment). It seems, therefore, that the effects of genes are simply additive. To their surprise, researchers found something else of relevance: certain topics appear to be of specific interest. For behavioral conduct including disorders, intelligence, or alcohol consumptions, there are hundreds of studies. New twin studies should focus on traits less well investigated, such as Multiple Sclerosis, epilepsy, or Parkinson’s Disease, says Posthuma in a press release of the university.
To demonstrate the balance between genes and environment for all the single traits of their meta-analysis for other researchers, the scientists have published a web tool (2) that allows us to identify traits that are less well investigated and may indicate a demand for research.
- 1. Polderman TJ, Benyamin B, de Leeuw CA, Sullivan PF, van Bochoven A, Visscher PM, Posthuma D. Meta-analysis of the heritability of human traits based on fifty years of twin studies. Nat Genet. 2015;47:702-9.
- 2. http://match.ctglab.nl/#/home
This is part 1 of a series covering twin health provided by Paul Enck from the Tübingen University Hospital and science writer Nicole Simon.