By: Natalie Wolchover
Published: 06/14/2012 12:02 PM EDT on Lifes Little Mysteries

Straight women are much more likely to get themselves knocked up than gay women. So, in terms of evolution, they would seem to have a better chance of passing on their genes, while at the same time it would seem that the genes that make women gay would quickly vanish from the gene pool. This raises the question, why are there gay women?

Lesbianism is indeed at least 25 percent genetic, as determined by a 2011 study of twins conducted in the United Kingdom. The study found that identical twin sisters (who share 100 percent of their DNA) are more likely to both be lesbians than are fraternal twin sisters (who share just 50 percent), proving that, all other environmental factors being equal, genes matter. While scientists have a theory for how male homosexuality propagates from one generation to the next, no one has yet produced a viable explanation for how the genes that promote lesbianism might do the same. [Why Are There Gay Men?]

A female's sexual orientation also appears to be partly influenced by her level of exposure to the male sex hormone androgen when she is in the womb. Greater hormone exposure correlates with more gender nonconformity early in her life (as a child, she may be called a "tomboy"), as well as a same-sex orientation later on. A study by Dutch psychologists published in the March issue of the Journal of Sexual Medicine reported that 10 to 12 percent of male and female children who feel discomfort with their gender go on to identify as gay or lesbian as adults. Meanwhile, just 1 or 2 percent of children who are comfortable with their gender identity turn out to be gay or lesbian.

Adding to the confusion about what causes lesbianism is the slipperiness of female sexuality itself. Unlike men, who are usually sexually oriented solely toward men or women, and whose sexuality is essentially fixed from puberty on, a decade of research by the University of Utah psychologist Lisa Diamond and others demonstrates that women have greater "erotic plasticity." Their sexual orientation can be shaped by cultural influences, altered by positive or negative experiences and intensified by feelings of love or attachment. Women are far more likely than men to "report remarkably late and abrupt onset of same-sex sexuality, often after heterosexual marriage," Diamond wrote in January in the Archives of Sexual Behavior.

Their sexual fluidity may emerge from the fact that, across the board, women are sexually aroused by images of both men and women (while men are typically only aroused by members of their preferred sex).

Therefore, the question "why are there gay women?" may be better worded as "why is female sexuality so fluid?" Plenty of women exist at both extremes of the straight-gay spectrum, but it is the formation of this slippery spectrum itself that most needs explaining. Evolutionary biologists have yet to determine what survival or reproductive benefit women's "erotic plasticity" confers. 

Follow Natalie Wolchover on Twitter @nattyover. Follow Life's Little Mysteries on Twitter @llmysteries. We're also on Facebook & Google+.

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Also on HuffPost:

GALLERY: 'Gay Households,' By State
Source: American Community Survey, 2009
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  • Wyoming

    558 same-sex couple households. The number of gay couples living together has increased in recent years, along with the increasing acceptance of homosexuality. But there are many more of these "gay households" in some parts of the country than others. Wyoming, with 558 of these households, has fewer than any other state. How does your state stack up? Which state has the most gay households? Keep clicking to find out... <em>The U.S. Census Bureau's 2005-2009 American Community Survey gives the most recent numbers for the total number of same-sex couple households in each state. These are absolute numbers, and don't represent the proportion of the states' households that are headed by same-sex couples</em>.

  • South Dakota

    802 same-sex couple households.

  • North Dakota

    875 same-sex couple households.

  • Alaska

    1136 same-sex couple households.

  • Montana

    1184 same-sex couple households.

  • Vermont

    2013 same-sex couple households.

  • Mississippi

    2028 same-sex couple households.

  • Nebraska

    2052 same-sex couple households.

  • Idaho

    2065 same-sex couple households.

  • Rhode Island

    2097 same-sex couple households.

  • West Virginia

    2259 same-sex couple households.

  • Delaware

    2677 same-sex couple households.

  • New Hampshire

    2966 same-sex couple households.

  • Utah

    3478 same-sex couple households.

  • Hawaii

    3507 same-sex couple households.

  • Arkansas

    3769 same-sex couple households.

  • Kansas

    4063 same-sex couple households.

  • New Mexico

    4172 same-sex couple households.

  • Iowa

    4299 same-sex couple households.

  • Maine

    4369 same-sex couple households.

  • Washington, D.C.

    4479 same-sex couple households.

  • Alabama

    4527 same-sex couple households.

  • Louisiana

    5083 same-sex couple households.

  • Oklahoma

    5438 same-sex couple households.

  • Nevada

    5504 same-sex couple households.

  • Connecticut

    6698 same-sex couple households.

  • Kentucky

    6934 same-sex couple households.

  • South Carolina

    7363 same-sex couple households.

  • Missouri

    9691 same-sex couple households.

  • Wisconsin

    9773 same-sex couple households.

  • Maryland

    10058 same-sex couple households.

  • Tennessee

    10377 same-sex couple households.

  • Massachusetts

    18263 same-sex couple households.

  • Indiana

    10948 same-sex couple households.

  • Oregon

    12258 same-sex couple households.

  • Colorado

    12558 same-sex couple households.

  • Virginia

    12795 same-sex couple households.

  • Michigan

    13512 same-sex couple households.

  • Arizona

    14375 same-sex couple households.

  • Washington State

    15290 same-sex couple households.

  • North Carolina

    17042 same-sex couple households.

  • New Jersey

    14838 same-sex couple households.

  • Georgia

    18263 same-sex couple households.

  • Minnesota

    10464 same-sex couple households.

  • Ohio

    21416 same-sex couple households.

  • Pennsylvania

    22086 same-sex couple households.

  • Illinois

    23397 same-sex couple households.

  • Texas

    39289 same-sex couple households.

  • Florida

    41847 same-sex couple households.

  • New York

    42618 same-sex couple households.

  • California

    81954 same-sex couple households.