There is no firm consensus about what percentage of the population is gay. Estimates range from 3.4-20%, though it is generally accepted that around 10% of the population is homosexual. While there is no definitive scientific answer as to what causes homosexuality, it is generally accepted that there are multiple contributing factors.
In looking at animal populations, homosexual behavior has been observed in 1500 animal species. There are even entire species that are bisexual. Sex is used in many populations to settle disputes and strengthen bonds, and about 5% of lifelong duck and goose partners are gay.
There is also observable science in human homosexuality. Finger length ratio and hair whorls are both indicators of homosexuality. Birth order also seems to play a big part in sexual orientation. Statistically, men who have older brothers are more likely to be homosexual, and it doesn’t even matter whether they grow up in the same home. Older sisters don’t appear to have any effect on whether a woman is gay. Check out this infographic for more.