Thank you for the fascinating question – it’s one that many scientists spend their entire careers exploring. When studying the etiology, or cause, of brain diseases, scientists must consider both nature and nurture. While these are often talked about as being separate (like the common saying “nature versus nurture”), it is the case that every brain disease is the result of an interaction between genes and the environment.

As you point out, brain diseases exhibit a high degree of heritability, meaning that the children of parents that have a brain disease are much more likely to have it as well. However, it often takes certain environmental factors to trigger the development of a brain disease.

Let’s consider an example. Imagine identical twins, which share 100% of the same genes, who have parents that have both suffered Major Depression. Because the disease runs in this family, it is likely the twins have the genes that predispose them to it as well. However this alone does not mean that both of the kids will develop depression – it still takes certain environmental factors to trigger the development of the disorder. For instance, suppose one of the twins consumes psychoactive drugs, which alter the chemistry of the brain. This makes it possible for one twin to develop depression and the other to not, even though they share the same exact genes that make it possible.

This example is greatly simplified. For any given brain disease, scientists have not determined all of the very subtle and massively complicated interactions between genes and the many environmental factors that exist. Because of this, scientists cannot often say who will get a particular brain disease and who will not. They advise, however, that we look into our family histories to get a sense of the potential genetic predispositions and then adjust the environmental factors under our control (like avoiding drug use) to try to prevent their development.

Thank you for the very interesting question!

There are a large variety of different brain diseases, some of which are more straightforward than others. The ones that you’ve mentioned, bipolar disorder and depression are certainly some of the most complicated diseases. Brain diseases can be caused by trauma, bacterial or viral infection, tumors, among other causes. However, bipolar and depression disorders are due mainly to defects in the brain neural circuits. Each brain cell can communicate with hundreds of other brain cells and these communications make a complicated. When the net isn’t properly formed, the brain doesn’t always send signals correctly so you have effects like depression.

Like many diseases, people get the disease due to a common of hereditary factors (such as family line) and random aspects from the environment. For bipolar disorder and depression, these diseases clearly run in families, but that doesn’t mean everyone in the family gets them. But if the disease does run in the family someone is more likely to get that disease. So if you have 2 people who may be just as likely to get depression, chances are one may get it and one may not. Or both may get it or neither may get it. In that sense there’s a degree of randomness with brain diseases.

Good question, and a huge amount of research is being devoted to understanding this right now. It's partly genetic, partly due to exposure to chemicals, experiences, etc.