For generations it’s been clear that a couple of major mental illnesses seem to occur much more frequently in some families than in others. But it hasn’t always been so easy to understand why such facts are the case. One problem is that it’s not easy to untangle the dual effects of genes and upbringing, of nature and nurture.
Identical twins share 100 percent of their genes, while fraternal twins share 50 percent. Generally, twins are raised in their households in very similar conditions.
Imagine John and Joe, who are identical twins in a bipolar family, and Ruth and Steve, who are fraternal twins in another bipolar family. If you are a researcher trying to separate nature versus nurture, you can comb through the medical records of lots of cases like John and Joe, as well as cases like Ruth and Steve. Now let’s say you look at all the times one of the identical twins has been diagnosed as bipolar. Next to that name you mark whether his or her identical twin has also been given a bipolar diagnosis. You do the same for the fraternal twins.
Studies of twins have been done in this way. The evidence is clear that the odds of being bipolar if your identical twin has the illness are high indeed. That’s not nearly so often the case if you have a fraternal twin who is bipolar. That’s good reason to conclude that genetics really matters for manic-depressive illness.
Like everything else about complex diseases, the twin studies don’t yield a clear-cut, 100 percent relationship. This suggests that developmental factors may play a role in bipolar disease. And although identical twins do share 100 percent of their genes, science is learning more and more about differences in ways genes are “expressed” and “silenced” – differences that may relate to environment.
Researchers have also studied children who were born into families with bipolar relatives but who were adopted shortly after birth by families without bipolar people in them. Just as with the twin studies, there’s imperfect but strong evidence that genetics has a lot to do with your chance of growing up to be bipolar. If your biological parents have the disease, even if you literally never saw them and were raised by non-bipolar parents, you stand a much greater than typical chance of being bipolar yourself.
Nature really matters. So it’s good advice to choose your biological parents with as much care as you can.
Today research is focusing on where in our DNA there lie the genes that create bipolar disease. I use the plural term “genes” because it’s quite clear that multiple different locations on our DNA strands are at issue.
Researchers in the 20th century moved from simple studies of twins and adoption to technically complex DNA work. This century, we can all hope, will move us substantially closer to more useful treatments for major mental illnesses.