Even as the number of Americans who support same-sex marriage reaches an all-time high, the ratio of Americans who believe that sexual orientation is innate to those who believe it is environmentally determined has remained relatively unchanged since the year 2000.
A Gallup Poll conducted earlier this month showed that 37 percent of Americans believe that being gay or lesbian is "due to factors such as upbringing and environment"; 42 percent think that people are born with their sexual orientation.
There is no consensus on the issue in today's scientific community. "Many think that nature and nurture both play complex roles," the American Psychological Association writes. "Most people experience little or no sense of choice about their sexual orientation."
There is evidence of physical differences between homosexual and heterosexual brains: A research project at the Karolinska Institute in Stockholm found that "the brains of gay men and women in some aspects are similar to the brains of those of the opposite sex." But those differences probably did not arise from environmental or genetic circumstances alone. Another study from the same institute examined differences between identical and fraternal twins in Sweden. It found that both genetic effects and the non-shared environment — that is, environmental differences in each twin's upbringing — had moderate effects on determining sexual orientation.