A massive new study has crunched the data of hundreds of thousands of siblings and twins in the United States in an effort to uncover the effects of genes and environment in over 500 different conditions. Targeting over 50,000 twin pairs, the research is the first of its kind to use large-scale data analysis in this way to evaluate the heritability of such a broad range of conditions.

"The nurture-versus-nature question is very much at the heart of our study," explains Chirag Patel, senior author on the new study. "We foresee the value of this type of large-scale analysis will be in shining light on the relative contribution of genes versus shared environment in a multitude of diseases."

The study homed in on over 700,000 sibling pairs, and over 56,000 twin pairs. In order to separate the effects of genes from environment, variable factors such as air pollution, climate, and socioeconomic status were accounted for using each individual's ZIP code.

The data was only able to track young twin pairs, up to the age of 24 years, and work from single snapshots in time (being based on data recorded in relation to health insurance claims through doctor or clinic visits). This means diseases that develop more slowly with age, such as neurodegenerative conditions, could not be accurately evaluated.

Despite these limitations, some of the more obvious results – such as lead poisoning being nearly entirely driven by environmental factors – affirm the value of the results and solidify the veracity of some of the more unexpected outcomes.

Obesity, for example, was found to be strongly driven by genetics, however when evaluating the data for morbid obesity it seems socioeconomic status plays a large role. The implication would be that while milder forms of obesity are underpinned primarily by genetics, it is the environment that plays a major role in the push up to cases of morbid obesity.

"This finding opens up a whole slew of questions, including whether and how a change in socioeconomic status and lifestyle might compare against genetic predisposition to obesity," says Patel.

Overall the researchers suggest around 40 percent of the studied conditions are primarily heritable, and 25 percent could be seen to be to be driven by social or environmental factors. Eye disorders and respiratory diseases showed the highest level of environmental influence, while cognitive disorders showed the highest level of genetic heritability, particularly conditions such as attention deficit hyperactivity disorder (ADHD).

It is clear that for the vast majority of conditions there is no single determining factor – instead there is a complicated interplay between genes and environment. The results are complex and dense, but the researchers created a web portal to allow any interested parties a way to dig through the mass of compelling correlations. The website is called CaTCH (Claims analysis of Twins Correlations and Heritability) and the ultimate goal is to help inform future research directions and health policy decisions.

"Our findings can provide signposts that inform subsequent research efforts and help scientists narrowly focus their pursuits," explains Chirag Lakhani, first author on the study. "For example, if our study of twins shows that there is very little heritability effect in a certain family of eye disorders, then future research should pursue alternative explanations."

Take a closer look at all the results on the CaTCH website.

The study was published in the journal Nature Genetics.