Sexual Orientation Determined in the Womb?

Sexual Orientation Determined in the Womb?

by Dr. Sadiq Muhammad


  1. Does Having Older Brothers Increase the Chances of Homosexuality?
  2. How Sexual Abuse in childhood can Trigger Homosexuality


  • Having more older brothers does put a male individual at a very slightly increased risk of homosexuality. This effect is thought to contribute to around 1 in 7 homosexual men. This does not apply to homosexual women.
  • There is no evidence this is as a result of any biological cause. The “maternal immune” theory, which states that the mother produces antibodies against male-specific targets with subsequent pregnancies, cannot be correct as we would expect to see gonadal and testicular disorders in homosexual men, as well as a higher incident of learning difficulties, neither of which we see.
  • We should note that having a greater number of older brothers is also associated with the development of homosexual paedophilia and also male-female homosexual transsexuals. Therefore any explanation for why homosexual men are more likely to have older brothers must similarly explain this association for these other two groups also.
  • The New Zealand Mental Health Survey demonstrated that homosexual individuals are at a much higher risk of “adverse events” in childhood such as witnessing or experiencing domestic violence, rape, sexual assault, death of a loved one, etc. Rape and sexual assault, in particular, are strongly associated with homosexual development. This effect shows a dose-effect relationship too; the more abuse, the greater the likelihood of non-heterosexuality.
  • That homosexual men (as well as homosexual paedophiles and male-female homosexual transsexuals) are more likely to have older brothers is, therefore, easily explained by the fact that with a greater number of older brothers, an individual would be at greater risk of exposure to predatory males seeking to sexually abuse a child, whether from within or outside the family. 

Does Having Older Brothers Increase the Chances of Homosexuality?

If the difference between homosexual and heterosexual identical twins lies not in the genetic differences, nor in the epigenetic differences (see: “Are People Born Gay?”) then what could be the cause? Researchers have made an interesting observation that seems to stand up to scrutiny: homosexual men are more likely to have older brothers.1 This is known as the Fraternal Birth Order observation. What could be the reason for this?

Having older brothers makes a man more likely to be gay. This is known as the Fraternal Birth Order observation

From the data collected, it seems that the more older brothers a man has, the more likely he is to become gay. The effect is not large, by any account, but it is there. It has been calculated to feature as one of the strongest likely associations to homosexuality for one in seven homosexual men2. Does this mean that the greater number of older brothers you have, the more likely you are to be gay? No. Such data cannot be extrapolated to an individual, for the simple reason that there may be (and indeed, are, as we shall demonstrate in due course) specific factors that are necessary to make this statistic applicable.

The explanation offered to this interesting observation is that as a mother has more sons, she develops antibodies against male-specific markers in the foetus’ brain. This antibody attack on male-specific brain regions results in the foetus developing an altered neurological state, which predisposes them to homosexuality, much like what we see with Rhesus antibodies in mothers.

The problem with applying the antibody-attack mechanism to homosexuality is that it is severely lacking in evidence. That brain regions between homosexual and heterosexual men and women do not differ is discussed in “Are People Born Gay”. Moreover, if the hypothesis was correct, there are other features we would expect to see in homosexual individuals, which we do not.

Firstly, if male-specific targets are under attack from maternal antibodies with increasing number of sons, we should expect to see certain abnormalities more prevalent in homosexual men than heterosexual men. As Dr. Whitehead explains3, if the attack is against male-specific targets then the testes should be attacked also. We should see a high prevalence of conditions such as hypospadias – a deformed penis resulting from low testosterone levels during development. We should also see poor sperm quality and a high prevalence of undescended testes. On the contrary, none of these conditions is found more in homosexual men. In fact, homosexual men have been shown, on average, to have larger male genitalia than heterosexual men4. They also show no evidence of a greater prevalence of learning difficulties, which would be likely if their developing brains were under immune attack from maternal antibodies. In fact, homosexual men actually show greater verbal fluency than heterosexual men.5 Moreover, no such region of the foetus has been found in men that are attacked by maternal antibodies and no antibody in women has been discovered to attack male-specific brain targets in the foetus. In other words, the theory remains just that – a speculative theory, lacking in evidence. 

Some believe that the Fraternal Birth Order observation has to do with intrauterine hormonal levels. This however has been debunked by William H. James.

Another method of approach to explain the Fraternal Birth Order observation is to attribute the increasing likelihood of homosexuality to changes or differences in intrauterine hormonal levels. On a superficial level, the claim seems to make intuitive sense. Some researchers have claimed, for example, that just as the likelihood of homosexuality increases with greater number of sons, the likelihood of having a son decreases. In other words, having had two sons, the likelihood of having a third son is not 50%, as it usually would be, but is less than 50%, indicating that a change in the hormonal in-utero environment may have occurred, predisposing the development of a girl rather than a boy. From this, it has been hypothesised that the in-utero environment was somehow “feminised” to produce homosexual sons rather than heterosexual ones.6 This observation was however demonstrated as false by William H. James who showed that this conclusion was drawn from statistical error.7 He pointed out that the largest data sets looking at 1.4 million children born between 1960 and 1994 demonstrate the very opposite – that the likelihood of having a son increases with the number of sons one has.

This subsequently raised the question that perhaps the Fraternal Birth Order observation can be accounted for by the fact that the maternal uterus becomes more masculinised with increasing number of sons. Once again, this appeared to have some support. For example, as mentioned in “Are People Born Gay”, some studies on finger-length ratios, indicating prenatal hormonal exposure, demonstrated that that homosexual men tend to have smaller finger-length ratios, indicating exposure to higher testosterone levels prenatally.8 From this observation, homosexual men seem to have been exposed to higher levels of testosterone than heterosexual men during in-utero development. Williams pointed out that this is exactly as one should expect if the Fraternal Birth Order observation is correct, because previous studies have shown that the gender of children is partially determined by the hormonal levels of the parents at the time of conception.9 It has been shown that higher levels of testosterone in the parents at the time of conception predisposes to the production of males rather than females.

Thus, it is to be expected that if homosexual men are more likely to have older brothers for reasons other than hormonal factors, then we are likely to also find that they have been exposed to higher testosterone levels, as a mother who is producing a number of sons will have higher testosterone levels. Williams explains this in the following manner:

…Homosexual men would be expected to be born into sibships characterized by a tendency to produce sons viz ex hypothesi those in which the parents (and, relevantly, the mother) have high testosterone levels.

In short, mothers who have higher testosterone levels are the confounding factor, since they are more likely to have sons, and if homosexuality is related to having older brothers for reasons other than hormonal influences, then that would explain why homosexual men have smaller finger length ratios. But why should homosexuality in men be associated with having greater numbers of older brothers, if not causally related to hormonal levels? Williams goes on to analyse post-natal factors and it is here that the strongest evidence is found to explain the Fraternal Birth Order observation.

How Sexual Abuse in childhood can Trigger Homosexuality

The key to explaining or understanding the Fraternal Birth Order observation is to realise that this observation is not only seen in homosexual men. This observation is also seen among homosexual male-female transsexuals10 and also male homosexual paedophiles (as compared to male heterosexual paedophiles)11. Thus, any explanation for the Fraternal Birth Order observation must also account for these other groups also.

One thing to be noted is that paedophiles who are exclusively or mainly homosexual, make up around 25% of all paedophiles.12 This rate of homosexuality among paedophiles is over ten times higher than the rate in the non-paedophile population. From this, it has been argued that paedophilia and homosexuality share some common causes.7 Evidence to support this can be found by the fact that both homosexuals and paedophiles report experiencing higher rates of child sexual abuse than heterosexual individuals. Male homosexuals have reported higher rates of child sexual abuse than male heterosexuals.13 Similarly, significantly higher percentages of homosexual and bisexual men report being encouraged or forced into sexual experiences prior to the age of nineteen.14 As regards paedophilia, there is a significant association between being a male victim of child sexual abuse and becoming a paedophile in later years.15 Some evidence has been interpreted to argue against such a correlation between homosexuality and child sexual abuse, however even in such studies, the rate of homosexuality among child-sexual abuse survivors was still found to have increased by 33%.16 It is important to emphasise here what these studies did not show: self-identifying homosexual individuals are not more likely to be paedophiles. Paedophiles may have a higher rate of homosexual tendencies but this does not make homosexual individuals more likely to have paedophilic tendencies.

Most recently, a National Survey in New Zealand provided data on 12,992 individuals, their sexual orientation and rates of five childhood adverse events.17 These five included: being badly beaten by a parent; witnessing other violence at home; sexual assault; rape; with the fifth adverse event being a composite of several types of other childhood traumas (such as the sudden death of a loved one, or a life-threatening vehicular accident). The individuals were classified into five groups as: 1) exclusively heterosexual; 2) heterosexual but having had same-sex experiences; 3) heterosexual but having had same-sex experiences and relationships; 4) bisexual; and 5) homosexual. The results showed that if an individual experienced three or more types of adverse events, such as domestic abuse, sexual assault, rape and others, they had a three times greater likelihood to later engage in homosexual acts or relationships in later life.18 The results further demonstrated that sexual assault and rape out of all the adverse events in childhood were the most likely to be associated with later homosexuality. The researchers also showed that the more adverse events in childhood an individual has, the less likely they are to be exclusively heterosexual, and the more likely they are to have engaged in homosexual acts or relationships (see Table 1).

Cumulated Adverse Childhood
Percentage Exclusively
Heterosexual (%)

Table 1: Homosexuality is associated with higher rates of adverse events in childhood: The greater number of adverse events an individual experiences the less likelihood there is that they will be exclusively heterosexual

Table 1 shows that a boy or girl who experiences three or more adverse events in childhood has a three times greater likelihood of engaging in homosexual acts as an adult. In other words, greater exposure to childhood traumas leads to a greater probability of homosexuality in later life and lower probability of heterosexuality.  In Table 4 of the study, we see stark correlations between adverse events prior to the age of 16 and later homosexual experience with or without homosexual relationships. Of the 12,992 individuals surveyed, 7,371 individuals reported suffering adverse childhood (prior to age 16) events. Being beaten at home was associated with a 2.8 times higher (CI 1.5-5.3) odds of having had homosexual experiences and relationships (p=0.0004).  Having suffered rape was associated with a 4.4 times higher (CI 2.3-8.3) odds of homosexual experiences and relationships (p<0.0001) and sexual assault was associated with a 5 times higher likelihood (CI 2.6-9.5) of homosexual experiences and relationships (p<0.0001).

This was a large, nationally representative survey that had the advantage of a high response rate for a community survey (73.3%) with very minimal non-response on questions regarding sexuality (<0.1%). Such results demonstrate a clear correlation between homosexuality and childhood adverse events. So how should we understand these results? Do they imply merely correlation, indicating that there is something else we are missing, or can childhood adverse events themselves cause the development of homosexuality?

It is difficult to argue from these results that this correlation does not imply a causal relationship. Some have argued that such sexual abuse in childhood is due to bigotry against “gay children” and that homophobic adults deliberately target gay children. This criticism holds no water, insofar as the average age of the abuse was far too young for children to self-identify as gay or to even have sexual emotions. In the New Zealand survey study, for example, the median age for sexual adverse events (rape or sexual assault) was nine years old; the median age for beatings was seven years old. Is it to be believed that nine-year-old children who have not even entered puberty are “gay” or “straight” and self-identify as such? Others have argued that those who will go on to engage in same-sex acts in later life are more likely to have such features in youth that make them desirable targets for sexual abuse. This however is unconvincing, as well. Indeed, it has been found that “prettier” boys are judged as more “effeminate” and are more likely to become homosexual in later life. This also holds true for girls who are judged as less attractive than others.19 As Williams points out however, unless one is to regard “prettiness” in boys as a feature that innately leads to homosexuality, it is reasonable to conclude that such boys or girls are more likely to be targeted for sexual abuse because of such features. Given the social isolation that such children often face from their gender groups as a result of their gender non-conforming appearances or mannerisms at a young age, it is very possible that sexual predators more easily target them.

It is true that correlation does not always imply causation. However, Hill’s Criteria for Causation is a good place to start.20 Firstly, the association between childhood adverse events and development of homosexuality in later life is strong. This is not a weak correlation picked out of much background noise but a clear and marked trend. Secondly, as an individual experiences more adverse events, the more likely they are to develop homosexual tendencies and behaviour. This meets the criteria for causality known as gradient. Thirdly, the findings have been replicated in many different studies, fulfilling the criteria of consistency21,22. Fourthly, the childhood adverse events seem to immediately precede the development of homosexuality; the median age for domestic violence was seven and for sexual assault and rape was nine, shortly prior to the average first homosexual attraction at ten years of age,23 which is the case cross-culturally and for both males and females. This fulfils the criteria of temporality – that the cause precedes the effect.

From the results of this study, the Fraternal Birth Order observation can be understood easily: the greater number of older brothers one has, the greater is the likelihood of child sexual abuse either from one’s own brothers or from the greater exposure to older males through one’s older siblings. Those with fewer older brothers will likely be less accessible to predatory older males than those with many. This is well supported by evidence from the Canadian Incidence Study of Reported Child Abuse and Neglect. This study looked at 135,573 cases of child maltreatment between 1998-1999 and demonstrated that siblings make up 29% of cases where the perpetrator of sexual abuse is a family member.24 Indeed, a survey of 796 college students in New England demonstrated that 15% of female students and 10% of male students had experienced sibling sexual contact.25 A quarter of these experiences were categorised as exploitative, either because of the large age gap between perpetrator and victim or because of the use of force. This explains the Fraternal Birth Order observation well and the fact that there are no other good explanations fulfils the criteria of specificity. This theory also fulfils the criteria of plausibility since the development of homosexual tendencies after such adverse events has sound basis in the reported experiences of homosexual individuals as well as in psychological theory. The criteria of analogy is also fulfilled since we see the same effect in the development of homosexual paedophilia, where child abuse has been found to be a specific and powerful risk factor for later development of paedophilia.26 In short, almost every criteria for causality is fulfilled and so an insistence on merely a correlational relationship between childhood adverse events and homosexuality is to ignore the body of evidence.

The Bradford Hill Criteria of Causality provide a method of assessing whether an association is likely to be causal or not.

Thus, the Fraternal Birth Order observation is well explained with reference to the role childhood adverse events may play in the development of homosexuality in adults, and the greater exposure to predatory older males having older brothers may bring.

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