Polderman & Posthuma publish largest twin study ever

19 May 2015

The study analyzed the results from 2,748 studies, containing nearly 18,000 human traits, and is based on information from more than 14.5 million twin pairs. The study is published on May 18, in the leading journal Nature Genetics.

Under supervision of Professor Danielle Posthuma (VU and VUmc) a small group of international scientists conducted the largest meta-analysis ever in the field of twin studies. The study analyzed the results from nearly 3,000 studies that were published between 1958 and 2012 worldwide. The impressive meta-analysis is based on information from more than 14.5 million twin pairs. The massive project took four years to complete. The study is published on May 18, in the leading journal Nature Genetics.

The results of the study provide insight into the relative contribution of genes and environment for hundreds of different traits. “By analyzing the results of all studies that examined the same trait, we obtain much more reliable estimates for the degree of heritability and the influence of the environment on that trait,” says Posthuma.

Because the study contains virtually all twin studies published so far, new conclusions could be drawn. “Thanks to the enormous sample size, we were able to compare heritability estimates between different age groups and between men and women. In general, we see that the heritability of traits decreases with age, and we see little evidence of differences between the sexes in heritability estimates”, clarifies VU-researcher Tinca Polderman, who is the first author on this study. The researchers also found that most twin studies are based on twin pairs from Europe, North America and Australia, while Africa and South America are virtually not represented.

The most studied traits are psychiatric and metabolic traits. “There appears to be a tremendously skewed distribution in traits that have been examined in twin studies. “We found hundreds of studies on behavioral disorders, intelligence and alcohol consumption, while just a few studies reported on e.g. multiple sclerosis, epilepsy or Parkinson’s disease. New twin studies on traits that have been examined extensively are actually redundant; future twin studies should focus on traits that have not been studied so far”, argues Posthuma.

The study also showed that for 2/3 of the examined traits the resemblance between monozygotic twin pairs is exactly twice as much as between fraternal twin pairs. “This means that a simple genetic model in which all the effects of genes can be added together underlies most of the traits. For 1/3 of all traits, this is not the case, and the effects of the individual genes are not independent of each other. The latter complicates the identification of genes, as for example in large genome-wide investigations, “explains Posthuma.

The researchers have made all their results available in an online tool that can be found on http://match.ctglab.nl. “On the website you can find the heritability of each trait that has ever been examined, for different age groups, for males and females separately, and across different countries”, states Polderman.

This study is important as a better a understanding of the variation in human traits is interesting for all kinds of disciplines: from medicine to psychology, and from social sciences to biology. It is expected that this paper will serve as an important reference for further research. In addition, Posthuma and her colleagues hope to end the eternal nature/nurture discussion. “When we take all traits together the average heritability 49%; the remaining 51% is environment”, so it’s really fifty-fifty, says Posthuma.

Polderman TJC, 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. Nature Genetics, May 2015, doi:10.1038/ng.3285