- Studies point to biological factors, but research is ongoing.

- Recent research backs up Alfred Kinsey's work from the 1940s showing a continuum of sexuality.

- Actor Cynthia Nixon recently stirred controversy when she declared that she had chosen to be gay.

Actor Cynthia Nixon recently stirred controversy when she declared that she had chosen to be gay.

The quote that prompted the debate? It appeared in a profile of Nixon in the New York Times Sunday Magazine:.

"I gave a speech recently, an empowerment speech to a gay audience, and it included the line 'I've been straight and I've been gay, and gay is better.' And they tried to get me to change it, because they said it implies that homosexuality can be a choice. And for me, it is a choice. I understand that for many people it's not, but for me it's a choice, and you don't get to define my gayness for me. A certain section of our community is very concerned that it not be seen as a choice, because if it's a choice, then we could opt out. I say it doesn't matter if we flew here or we swam here, it matters that we are here and we are one group and let us stop trying to make a litmus test for who is considered gay and who is not."

In the aftermath of Nixon's take on her sexuality, a discussion about the origins of homosexuality has been reignited. The Sex in the City star raised questions about the idea of sexual fluidity and prompted some to ask questions about how we think of sexual orientation.

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Many scientists, though, have concluded that sexuality is at least partly determined by biology.

"Biology tells us that sexual orientation is strongly influenced by prenatal hormonal and genetic factors and there is little information supporting the notion that education or interactions with peers play any substantial role," said Jacques Balthazart of the University of Liege in Belgium, and author of The Biology of Homosexuality. "This being said, the part of the variance explained by biology is still limited and there is possibly room for personal choice. In addition, multiple factors probably contribute to determine homo versus heterosexuality and this could vary from one individual to another."

One landmark series of studies on twins in the early 1990s, for example, showed that if one identical twin is gay, the other has a 50 percent chance of also being homosexual. Psychologist Michael Bailey of Northwestern University's work showed that among fraternal twins (who do not share the same DNA), the probability drops to 20 percent.

Other studies have noted specific detailed differences between people with different sexual orientations: for example, a 2011 study published in Frontiers in Neuroendocrinology showed that oto-acoustic emissions in the inner ear are more frequent in women -- unless the woman is lesbian. In animals treated with androgenic hormones as embryos, the emissions also lessen, leading researchers to believe that prenatal hormones play some role in sexual identity.

Cynthia Nixon's recent statements about choosing to be gay caused an uproar for some in the gay community. Corbis

Bisexuality and the idea of sexual fluidity further complicate the science of sexuality. In Nixon's case, her idea of choice "probably corresponds more to bisexuality," Balthazart hypothesized (as have others).

"Theoretically, one can interpret this condition as resulting from the same biological factors as those that determine homosexuality but that would not have been fully active," Balthazart said. "This is, however, a theoretical construct and there are little or no data to support this interpretation. Then once you are 'biologically' bisexual, I am convinced that is is a personal choice to have sexual relationships with partners of one sex versus another."

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Lisa Diamond's 2008 book, "Sexual Fluidity: Understanding Women's Love and Desire" brought attention to the idea that sexuality isn't always stagnant.

"When people who have some media attention say things about gay being a choice it really reverberates in the sexual minority community," said Ellen Schecter, a pyschologist in Hanover, New Hampshire, who specializes in sexual identity and identity. "It's interesting to think about what is talking about when she says choice. Did she choose this woman? Did she choose to fall in love, or did she choose to honor her feelings for this woman?"

Schecter wrote her dissertation on long-time lesbians who partnered with men in midlife.

"Research on sexual fluidity shows that while most people have a stable sexual orientation, some of us do not," Schecter said. "Sexuality can change over time. Now, that makes it sounds like on Wednesdays I could be attracted to guys and on Fridays, girls, but it's a much more complicated phenomenon. Most people who don't study in this field define sexual orientation by sexual behavior: if you have sex with a man and you're a man, you're gay. But in fact, as the study of sexual orientation has developed, there's more than actual behavior: there's attraction, desire, fantasy..."

People on both ends of the spectrum often feel threatened by the idea of sexual fluidity, Schecter said.

"We're really driven to put things in boxes," she said. "It makes us more comfortable. It's hard to sit easily with ambiguity and the grey areas of life."

Like many who study in the field, Schecter does not feel being gay is a matter of choice.

"You feel what you feel," she said. "What you do with those feelings might be a choice."

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Some say the debate shouldn't matter outside of the scientific world: Despite his conviction that sexuality is predetermined to some degree, Balthazart thinks that society shouldn't need scientific proof to respect all types of sexual orientation.

"Society should accept any kind of sexual orientation independent of the interpretation (biology versus choice)," he said. "The fact is, however, that the present Western societies will probably show more tolerance towards gays and lesbians if their orientation is determined by prenatal biology -- and I think that data strongly argue in that direction."

That statement hints at the fear behind the anger expressed by sexual minorities this week. Such an angry reaction from the community surprised Tracy Baim, editor of Windy City Times, a GLBT newspaper in Chicago.

"I knew this issue was controversial in the community, but the anger was a little bit shocking," said Baim, who defended Nixon's comment in a Huffington Post article. "Some people haven't really thought out the historical precedence. People used to look at brain sizes to justify racism…I think the pursuit of science is great, but you have to be careful with the application. If it's genetic, they could 'cure' us with a pill."