Female identical twins, in their early twenties—one is diabetic, the other is not. Male identical twins, in their late teens—one has rheumatoid arthritis, the other does not. But how can this be? When twins share a DNA profile, isn’t their future mapped out? Isn’t it just a matter of time until both twins get sick?

The surprising answer to this question is no, in most cases. Most diseases depend on both genetic and environmental factors to cause the disease to manifest itself. In many diseases, the environmental factors haven’t been identified, or are fuzzy. This is where twin studies on diseases can be the most helpful.

Identical twins share the same DNA, and in most cases, the same environment until they are eighteen years old. But they also experience differences in their environment. One twin may get injured playing sports, for example, while the other one is not. One twin may eat a lot of chocolate and be obese, when the other doesn’t eat it at all and is slender. One twin may have had the mumps as a child, while the other one avoided the disease. All of these environmental factors can possibly influence the emergence of disease later in life.

In a study published in Arthritis and Rheumatism, researchers found that if one identical twin contracted rheumatoid arthritis, the other twin only had a 15% chance of developing the disease. According to the American Diabetes Association, if one identical twin develops diabetes, the other twin has, at most, a 50% chance of developing the disease, and can possibly avoid it by taking measures to improve health.

Similarly, a recent Swedish study on men 65 and over found that Alzheimer’s Disease is very much influenced by genetics, though environmental factors seem to be at work as well. In that study, if one twin develops Alzheimer’s Disease, the other twin has a 45% chance of developing the disease.

A long-term study of World War II veterans who were twins found no difference in the rates of cancer among identical and fraternal twins, which led researchers to the conclusion that many cancers are more influenced by lifestyle choices than genes.

In contrast, a Mayo Clinic study done in 2000 found that fraternal twin women had more than twice the normal risk of developing post-menopausal breast cancer than other types of twins or singletons. This finding led researchers to hypothesize that maternal levels of estrogen (which is higher in female fraternal pregnancies than female identical pregnancies) impacted breast cancer likelihood for the women later in life—an amazing idea.

The message that most of these studies are sending is that even if a disease has a large genetic component, environmental factors, many that are within our control, usually play just as large of a role. Twins, both identical and fraternal, have helped researchers learn much about the role of genetics and environment in disease development, and we should thank them for it.