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Are the cause(s) of sexual orientation due to nature or nurture?
Three proofs that homosexual & bisexual sexual orientations
do NOT have a genetic cause, with rebuttals

Part 6:
Proof #2, based on the birth rate
of lesbians and gays, with a rebuttal.

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This topic is continued from the previous essay.

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One frequently cited proof that sexual orientation is not genetically caused is based on the birth rates of lesbians and gays:

Lesbians and gays have, on average, far fewer children who are genetically related to them than do "straight" persons. Thus if a person were born with an allele -- a variety of gene -- that caused them to be lesbian, gay, or bisexual (LGB) they would, on average, conceive fewer, genetically related children during their lifetime. Most probably would not pass pass their "gay gene(s)" at all to the next generation. If a "gay gene" existed, copies of it within the population could be expected to become fewer with each generation, and would eventually become quite rare. "Straight" couples would simply outbreed same-sex couples to the point that the LGB community would eventually disappear.

However, lesbians and gays are still with us. They appear to be present at about 5% of the population in every country and in throughout every era in history. Many religious and social conservatives cite the perseverance of the LGB population as proof that homosexual or bisexual orientations cannot be genetically caused. They conclude that there must be something in the environment that is causing homosexuality and bisexuality.

Many causes have been suggested:

  • The type of parenting a child receives;

  • Being sexually molested as a child;

  • A person choosing to be a lesbian or gay, typically during their pre-teen or teen years;

  • Experimenting with same-gender sexual behavior and becoming addicted to it;

  • etc.

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How often do gays and lesbians pass on their genes directly to their children?

We have not been able to find information on this. However, there are indications that the percentage is small.

The Total Fertility Rate (TFR) of a country is expressed as the number of live births per 1,000 women who are in the age range of 15 to 44 years. The TFR for the U.S. in recent years has been about 1.9 babies per woman during her lifetime. A TFR of slightly more than 2 is required to stabilize the population, assuming that the levels of immigration and emigration are equal. 1 Thus American women -- the vast majority of whom are heterosexual -- are having a sufficient number of births so that, assisted by immigration, the U.S. population is slowly increasing.

The 2010 U.S. Census data shows that the percentage of same-sex households who are raising children is about 19% in Delaware, 20% in Pennsylvania, 21% in California, 26% in Kansas, and 28% in Wyoming. 2 This compares to about 42% of opposite-sex households nationally which are raising children. This is almost twice the rate of same-sex couples.

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Proof #2 (Continued):

Same-sex couples are infertile; they cannot conceive children by themselves. In many cases, same-sex couples build their families by adopting children. Less commonly:

  • Some females in same-sex relationslhips, who are lesbians and/or bisexuals, will undergo intrauterine insemination (IUI) or in vitro fertilization (IVF) using donated sperm in order to conceive a child. This will pass their "gay gene(s)" onto the next generation.

  • Some male same-sex couples, composed of gays and/or bisexual male(s), will obtain the help of a gestational carrier. This is a woman who would conceive using the sperm from one of the males or perhaps a mixture of sperm from both males.

Considering that:

  • The percentage of same-sex couples who are raising children is only about a half that of opposite-sex couples, and

  • That many of these children in families headed by same-sex couples were adopted, and thus are genetically unrelated to their parents,

then one can conclude that a relatively small minority of gays and lesbians are passing their genes on to the next generation. Perhaps 15% would be a reasonable guess. Unfortunately, there does not seem to be a national agency that collects annual data on the number of IUI and IVF procedures, and the number of gestational carriers giving birth. So a guess is all we have to go on at this time.

In 2013, the U.S. population was 316 million. 3 Assuming that 5% of the population are gay or lesbian, then the number Americans who identify as gay or lesbian, -- or who will identify that way in the future -- is currently about 16 million. If our 15% value is correct, then the number who pass on their genes to the next generation would then be about 2.5 million. These 2.5 million would subsequently mature and pass on their genes to perhaps a third of a million children. There would be significantly fewer with each generation. In a century, there would be very few members of the LGB community left in the country.

However, no such drastic reduction among the LGB community during each generation has been observed in the past. It does not seem to be happening currently. Therefore, many people believe that somehow, new gays, lesbians and bisexuals are being caused by non-genetic means. This means that they are made, not "born that way." They conclude that the cause of homosexuality is not genetic. If it is not genetic then they assume that it would be more easily changed during therapy.

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The beginnings of a rebuttal of the "birth rate" argument against a genetic cause of homosexual orientation:

However, researchers into genetics and human sexuality have reached a near consensus that sexual orientation does have one or more genetic causes. Dozens of studies comparing hand shapes, birth order, eye blinking, hearing sensitivity, ear emissions, pheromones, brain structure, etc. of heterosexuals, lesbians, and gays have supported this belief. The most definitive studies have involved studying identical and fraternal twins in which at least one twin is lesbian or gay. Many researchers no longer hold a single gene to be responsible; they suspect a person's sexual orientation is determined by the interaction of multiple genes.

However, one "fly remains in the ointment." It is called the "Darwinian paradox." It exists and has been a mystery. Professor Ciani A Camperio et al, at the University of Padua in Italy asks:

"... if male homosexuality has a genetic component and homosexuals reproduce less than heterosexuals, then why is this trait maintained in the population?" 4

There is one phenomenon in genetics that may be responsible for at least part of the perseverance of lesbians and gays in the population. It is called "balanced polymorphism." This involves an allele (one variation of a gene) that causes an advantage in some situations, even as it simultaneously causes a disadvantage in others.

One example of balanced polymorphism involves the allele that causes sickle cell anemia in humans. This is a serious blood disease that can be fatal. A baby who is born with two of the sickle cell alleles may develop the disease. However, if a child is born with only one sickle cell allele, they will not develop the disease and they will have resistance against malaria. In malarial regions of the world, over time, this allele has become more common even though it causes the incidence of sickle cell anemia to increase.

Professor Camperio's research team at the University of Padua in Italy looked for and found a similar balanced polymorphism phenomenon among gay males.

The abstract of their article states:

"In a sample of 98 homosexual and 100 heterosexual men and their relatives (a total of over 4600 individuals), we found that female maternal relatives of homosexuals have higher fecundity than female maternal relatives of heterosexuals and that this difference is not found in female paternal relatives. The study confirms previous reports, in particular that homosexuals have more maternal than paternal male homosexual relatives, that homosexual males are more often later-born than first-born and that they have more older brothers than older sisters. 4

The researchers concluded that although gays have fewer genetically related sons and daughters of their own, their mothers, aunts, etc. tend to have more children. And so the "gay allele(s) continue from generation to generation.

In 2012, Christine Hsu, writing in Medical Daily, said:

"Evolutionarily speaking, homosexuality as a trait would not last because it discourages reproductive sex with women and therefore procreation.

However a new study, published in The Journal of Sexual Medicine, found a correlation between gay men and their mothers and maternal aunts, who are prone to have significantly more children compared to the maternal relatives of straight men. 7

Researchers led by Andrea Camperio Ciani, from the University of Padua in Italy, say that the findings of the link between homosexuality and female fertility strongly support the "balancing selection hypothesis," which suggests that a gene which causes homosexuality also leads to high fecundity or reproduction among their female relatives.

The team noted that the "gay man gene" may not get passed down directly, but instead survive through the generations through future generations making their male inheritors gay." 5

Meanwhile, Paul Vasey, a Canadian psychologist from the University of Lethbridge in Alberta, is conducting research into homosexuality in Samoa. Samoa is a very religious country in which about 98% of the population follow various denominations of Christianity. 6 Same-gender sexual activity is considered a criminal act there. However, Samoan society generally accepts a type of "third gender" -- biological males called fa'afafine. The term translates as "in the manner of a woman."

He has uncovered some trends that are similar to those found by the Padua researchers. He found two factors that improve the probability of fa'afafines propagating their genes through their nieces and nephews, even though few of them have genetically-related children themselves. When compared to heterosexual males, the fa'afafines:

  • Invest greater effort in helping out their nieces and nephews, thus increasing the latter's probability of survival.

  • Have on average a significantly greater number of children on both sides of the family. 7

To our knowledge, no similar studies have been conducted yet for lesbians in Italy, Samoa or elsewhere in the world.

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This topic continues in the next essay

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References used:

The following information sources were used to prepare and update the above essay. The hyperlinks are not necessarily still active today.

  1. "2011: National Vital Statistics Reports, Vol. 62, No. 1," Page 40, Table 12, Centers for Disease Control, 2013-JUN-28, at:
  2. Susan Donaldson Janes, "Census 2010: One-Quarter of Gay Couples Raising Children," Good Morning America, 2011-JUN-23, at:
  3. "State & County QuickFacts," U.s. Census Bureau, 2014-DEC-03, at:
  4. Andrea Camperio-Ciani, et al, "Evidence for maternally inherited factors favouring male homosexuality and promoting female fecundity," Proceedings of the Royal Society/ Proceedings of Biological Sciences, 2004-NOV-07, at:
  5. "'Gay Gene' survived evolution as it is carried by mothers who have more children, study," Medical Daily, 2012-JUN-13, at:
  6. "Religion in Samoa," Wikipedia, as on 2014-OCT-15, at:
  7. "Survival of the Fabulous," 2013 episode of the Canadian Broadcasting Corporation TV program on "The Nature of Things."

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Copyright 2014 by Ontario Consultants on Religious Tolerance
Original posting: 2014-DEC-15
Latest update: 2014-DEC-27
Author: B.A. Robinson

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