The identical twin of a gay man has a 50 percent chance of being gay, but the fraternal twin of a gay man has only a 15 percent chance of being gay. This suggests that:

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The fact that identical twins, who share all of their genetic material, have a 50 percent chance of both being gay suggests that genetics play a significant role in sexual orientation. However, the fact that fraternal twins, who share only about 50 percent of their genetic material, have a much lower chance of both being gay suggests that environmental or other non-genetic factors also play a role. Therefore, genetic factors play a large role in influencing sexual orientation in men, but they do not completely determine it.

The Identical Twin of a Gay Man Has a 50