By: Natalie Wolchover
Published: 06/12/2012 12:59 PM EDT on Lifes Little Mysteries

While female sexuality appears to be more fluid, research suggests that male gayness is an inborn, unalterable, strongly genetically influenced trait. But considering that the trait discourages the type of sex that leads to procreation — that is, sex with women — and would therefore seem to thwart its own chances of being genetically passed on to the next generation, why are there gay men at all?

Put differently, why haven't gay man genes driven themselves extinct?

This longstanding question is finally being answered by new and ongoing research. For several years, studies led by Andrea Camperio Ciani at the University of Padova in Italy and others have found that mothers and maternal aunts of gay men tend to have significantly more offspring than the maternal relatives of straight men. The results show strong support for the "balancing selection hypothesis," which is fast becoming the accepted theory of the genetic basis of male homosexuality.

The theory holds that the same genetic factors that induce gayness in males also promote fecundity (high reproductive success) in those males' female maternal relatives. Through this trade-off, the maternal relatives' "gay man genes," though they aren't expressed as such, tend to get passed to future generations in spite of their tendency to make their male inheritors gay.

While no one knows which genes, exactly, these might be, at least one of them appears to be located on the X chromosome, according to genetic modeling by Camperio Ciani and his colleagues. Males inherit only one X chromosome — the one from their mother — and if it includes the gene that promotes gayness in males and fecundity in females, he is likely to be gay while his mom and her female relatives are likely to have lots of kids. If a daughter inherits that same X-linked gene, she herself may not be gay, but she can pass it on to her sons. [Why Are There Gay Women?]

But how might the "gay man gene" make females more reproductively successful? A new study by Camperio Ciani and his team addresses the question for the first time. Previously, the Italian researchers suggested that the "gay man gene" might simply increase androphilia, or attraction to men, thereby making the males who possess the gene homosexual and the females who possess it more promiscuous. But after investigating the characteristics of 161 female maternal relatives of homosexual and heterosexual men, the researchers have adjusted their hypothesis. Rather than making women more attracted to men, the "gay man gene" appears to make these women more attractive to men.

"High fecundity, that means having more babies, is not about pleasure in sex, nor is it about promiscuity. The androphilic pattern that we found is about females who increase their reproductive value to attract the best males," Camperio Ciani told Life's Little Mysteries.

Turns out, the moms and aunts of gay men have an advantage over the moms and aunts of straight men for several reasons: They are more fertile, displaying fewer gynecological disorders or complications during pregnancy; they are more extroverted, as well as funnier, happier and more relaxed; and they have fewer family problems and social anxieties. "In other words, compared to the others, [they are] perfect for a male," Camperio Ciani said. Attracting and choosing from the best males enables these women to produce more offspring, he noted.

The new study will appear in an upcoming issue of the Journal of Sexual Medicine.

Of course, no single factor can account for the varied array of sexual orientations that exist, in men as well as in women. "It is quite possible that there are several influences on forming a homosexual orientation," said Gerulf Rieger, a sexual orientation researcher at Cornell University. He noted that environmental factors — including the level of exposure to certain hormones in the womb — also play a role in molding male sexuality. But as for why genetic factors would exist that make men gay, it appears that these genes make women, as well as gay men, alluring to other men.  

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

Copyright 2012 Lifes Little Mysteries, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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.