Twins, even if they're identical, can be very different personality-wise — physically, not so much. Unfortunately, it seems like developing cancer may be one of the qualities both identical and fraternal twins share. Having a twin sibling who has been diagnosed with cancer poses excess risk for the other twin to develop any form of cancer, according to a new, large study.

Led by researchers from the Harvard T.H. Chan School of Public Health, the University of Southern Denmark, and the University of Helsinki, the research examined 23 different types of cancer and found excess familial risk for almost all of them, including both common cancers (like breast and prostate cancers) and uncommon cancers (like ovarian, stomach, and testicular cancers).The study is the first to provide estimates for family risk for these cancers, all the while demonstrating an interesting finding — in twin pairs where both developed cancer, each twin often developed a different type of cancer, suggesting that, in some families at least, there is a shared increased risk of any type of cancer.

"Prior studies had provided familial risk and heritability estimates for the common cancers — breast, prostate, and colon — but, for rarer cancers, the studies were too small, or the follow-up time too short, to be able to pinpoint either heritability or family risk," explained Lorelei Mucci, associate professor of epidemiology at Harvard Chan School and co-lead author of the study, in a statement.

The study first examined familial risk of cancer, a measure of an individual’s risk of cancer, as well as the heritability of cancer — the measure of how much variation in cancer risk is due to genetic factors. More than 200,000 twins were examined for the study, and both identical and fraternal twins were represented. The inclusion of fraternal twins was particularly important, since they're genetically similar to siblings who aren't twins, which is something that can provide information about an increased risk for cancer in families where one sibling is diagnosed.

"Findings from this prospective study may be helpful in patient education and cancer risk counseling," said Jaakko Kaprio, from the University of Helsinki and co-senior author of the study.

The twins were followed over an average of 32 years for the study. Overall, about one third of the people in the study developed cancer over the course of a lifetime. Of the twin pairs who both developed cancer, the same cancer was diagnosed among 38 percent of the identical twins and 26 percent of the fraternal twins. The researchers then estimated that for identical twins, when one was diagnosed with cancer, the co-twin's risk jumped to 46 percent. Among fraternal twins, the co-twin's risk of developing cancer was 37 percent.

It was noted that the overall heritability of cancer was 33 percent, with significant heritability found for skin melanoma (958 percent), prostate cancer (57 percent), non-melanoma skin cancer (43 percent), ovarian cancer (39 percent), and kidney cancer (38 percent).

"Because of this study's size and long follow-up, we can now see key genetic effects for many cancers," said Jacob Hjelmborg, from the University of Southern Denmark and co-lead author of the study.

Source: Mucci L, Hjelmborg J, Harris J, Czene K, Havelick D, Scheike T, et al. Familial risk and heritability of cancer among twins in Nordic countries. JAMA. 2016.