For the first time, scientists have identified genetic variations associated with human bisexual behavior — and found these markers are linked to risk-taking and having more offspring when they are carried by heterosexual men.

Jianzhi "George" Zhang, a professor at the University of Michigan and senior author of the new research, said it helped answer the long-standing evolutionary puzzle of why natural selection has not eliminated the genetics underpinning attraction within the same sex.

The study, published Wednesday in Science Advances, was based on data from more than 450,000 people of European descent who signed up for the U.K. Biobank, a long-term genomics project that has proven a major boon for health research.