A study attempting to clarify the ongoing 'nature vs nurture debate' has confirmed that the issue is complex - and that most conditions are probably caused by an interaction of both.
The new study, published in Nature Genetics, is thought to be the largest twin study to date, and investigated the health of 56,396 pairs of twins in addition to another 724,513 siblings across the United States. It found that about a quarter of the 560 diseases seen were partly influenced by the environment.
The authors hope that their findings will help direct future research into the causes of disease by informing scientists about the most appropriate focus to take.
'For example, if you're interested in lead poisoning, genetics plays a very small role and we need to think about the environment,' said first author, Dr Chirag Lakhani at the Blavatnik Institute, Harvard Medical School. 'But for other cases like ADHD that are more likely to have a hard genetic component, we can think about other ways to interrogate the disease.'
The study used data from the private insurance company Aetna. As this included the participants' zip codes, the researchers were able to predict factors like socioeconomic status and exposure to air pollution, and combine this with information about doctor visits and disease diagnoses.
While some diseases are known to be caused entirely by a person's genetics, this study confirmed that many of the 560 investigated conditions seen in the study are neither purely genetic nor purely environmental but arise as a result of a complex interplay between the two.
Overall, genes influenced at least 40 percent of the 560 diseases, and around one quarter of conditions were partly influenced by environmental conditions.
However, and as expected, the degree to which genetics and the environment contribute to disease depends on the precise disease being looked at. For example, cognitive conditions such as attention-deficit/hyperactivity disorder had the strongest genetic influence, while eye disorders and respiratory diseases were more influenced by the environment.
The study also found that both genes and environment contribute to the cost of care, allowing nearly 60 percent of monthly health spending to be predicted by analysing genetic and environmental factors. The authors therefore believe the results can help forecast long-term spending for various diseases and inform resource allocation and policy decisions.
The study does have some limitations. In particular, researchers did not have information on which twins were identical (and therefore share 100 percent of their DNA) and which were fraternal (who share only 50 percent of their DNA).
Furthermore, only twins younger than 24-years-old were studied and the work was not designed to follow disease development over time. This means more work is still needed to better assess the genetic and environmental influences of diseases that tend to develop in middle or old age, such as heart disease and neurodegenerative conditions like Alzheimer's.
Dr Melody Goodman, a biostatistician at New York University's College of Global Public Health, has also raised caution that because the study represented a commercially-insured population, it was unlikely to fully capture how poverty can contribute to poor health.
The authors have noted, however, that the next step will be to repeat the study design using many different databases. The data and more information can be found here.