Curtis24
09-06-2010, 02:40 AM
From what I"ve read, many anthropologists comment on the phenomena of "reemergence", whereby a numerically inferior racial type suddenly increases in great number. Many assumed this was due to natural selection - the situation changed so that the minority type suddenly had an advantage, and began multiplying rapidly, whereas other racial types had a disadvantage with the new situation, and multiplied less.
Yet, I've read on many forums about a new phenomena in Europe, especially Eastern Europe, whereby brachcephals are increasingly having dolichocephal children. Why is this happening?
Consider this theory, outlined in the articles below. I know its a lot of text, but I can't succinct it better than the author does.
The genius of Robert L. Trivers
It is commonly believed that whether parents conceive a boy or a girl is entirely up to chance. Close enough, but not quite. It is largely up to chance, but there are factors that very subtly influence the sex of an offspring. It is also commonly believed that exactly half the babies born are boys and the other half are girls. Close enough, but not quite. The normal sex ratio at birth is .5122 – 105 boys for every 100 girls. But the sex ratio varies slightly in different circumstances and for different couples. So what factors affect the sex of the child?
Any discussion of sex ratio at birth must begin with the work of Robert L. Trivers, who is one of the greatest evolutionary biologists of his generation. In 1973, Trivers teamed up with a mathematician, Dan E. Willard, to formulate one of the most celebrated principles in evolutionary biology, called the Trivers-Willard hypothesis. The hypothesis states that wealthy parents of high status have more sons, while poor parents of low status have more daughters. This is because children generally inherit the wealth and social status of their parents. Sons from wealthy families, who themselves become wealthy, have, throughout most of evolutionary history, been able to expect to have a large number of wives, mistresses, and concubines, and produce dozens or hundreds of children, whereas their equally wealthy sisters can have only so many children. So wealthy parents should “bet” on sons rather than daughters.
Conversely, poor sons can expect to be completely excluded from the reproductive game, because no women would choose them as their mates. But their equally poor sisters can still expect to have some children if they are young and beautiful. The “fitness ceiling” – the best one can do reproductively – is much higher for men than for women, but the “fitness floor” – the worst one can do reproductively – is much lower for men. So poor parents should “bet” on daughters rather than sons. Natural selection designs parents to have a biased sex ratio at birth depending upon their economic circumstances – more boys if they are wealthy, more girls if they are poor.
There is evidence for this hypothesis throughout human societies. American Presidents, Vice Presidents, and cabinet secretaries have more sons than daughters (although only during the first half of its history). Poor Mukogodo herders in East Africa have more daughters than sons, both at birth and in the 0-4 age group. Church parish records from the 17th and 18th centuries in Germany show that wealthy landowners in Leezen, Schleswig-Holstein, had more sons than daughters, while farm laborers and tradesmen without property had more daughters than sons. Among the Cheyenne Indians on the American Plains, prestigious, high-status “peace chiefs” have more sons than daughters, while poor and marginal “war chiefs” have more daughters than sons in the 0-4 age group. In the contemporary United States and Germany, the elite – judged by the listing in their respective country’s Who’s Who – have a greater proportion of sons among their offspring than does the population in general. In an international survey of a large number of respondents from 46 different nations, more wealthy individuals are more likely to indicate a preference for sons if they could only have one child, whereas less wealthy individuals are more likely to indicate a preference for daughters. While there is some counterevidence, most evidence is in support of the Trivers-Willard hypothesis.
In my next post, I will discuss how Trivers’s genius in the Trivers-Willard hypothesis has been extended to new directions.
Extending Trivers’s genius
In my previous post, I introduce the Trivers-Willard hypothesis, which predicts that, relative to the baseline offspring sex ratio of 105 boys for 100 girls, wealthy families are more likely to have sons while poor families are more likely to have daughters. More recently, there has been a theoretical extension of the original Trivers-Willard hypothesis, called the generalized Trivers-Willard hypothesis.
The idea behind the new hypothesis is the same as that behind the old one, but it extends the idea to many other factors besides the family’s wealth and status. The new hypothesis suggests that if parents have any trait they can pass on to their children that is better for sons than for daughters, then they will have more boys. Conversely, if parents have any trait they can pass on to their children that is better for daughters than for sons, then they will have more girls. Parental wealth and status are just two of the traits they can pass on to their children that are more beneficial for sons than for daughters, but there are many other factors.
It is important to note that neither the original Trivers-Willard hypothesis nor the generalized Trivers-Willard hypothesis predicts that parents with a given set of heritable (and immutable) traits will produce offspring of only one sex or the other. Assume that the sex of a child is determined by a toss of an imaginary coin, and the “normal” or baseline parent has an “unbiased” coin that comes up “boy” 51.22% of the time and comes up “girl” 48.78% of the time (to reflect the “normal” baseline secondary sex ratio of 105 boys to 100 girls). If a parent possesses some heritable trait that is better for sons than for daughters, then the coin becomes “biased” in favor of sons and now comes up “boy,” say, 55% of the time. Such a parent is therefore significantly more likely to have a boy than the “normal” or baseline parent. However, it does not mean that such a parent will only have boys. It is still possible (albeit less likely than for others) for a parent with a probability of having a boy of .55 to have a girl, or even three girls in a row.
Brain types are a good example of heritable trait which affects boys and girls differently. Strong “male brains,” which are good at systemizing (figuring things out), are more beneficial for sons than for daughters, while strong “female brains,” which are good at empathizing (relating to people), are more beneficial for daughters than for sons. Since brain types are heritable, the generalized Trivers-Willard hypothesis would predict that parents with strong male brains, such as engineers, mathematicians, and scientists, are more likely to have sons, while those with strong female brains, such as nurses, social workers, and schoolteachers, are more likely to have daughters. This indeed appears to be the case. While the sex ratio at birth among the general population is .5122 – 105 boys for every 100 girls – the data show that the sex ratio among engineers and other systemizers is .5833, that is, 140 boys for every 100 girls. The comparable sex ratio among nurses and other empathizers is .4255, that is, 135 girls for every 100 boys.
By the same token, as I mention in a previous post, tall and big parents have more sons and produce more male fetuses (because body size was a distinct advantage in male competition for mates in the ancestral environment, while body size has no particular advantage for women), and short and small parents have more daughters and produce more female fetuses. (And, as I explain in another post, this observation can potentially explain why more boys are born during and after major wars.) Because violence was probably a routine means in the male competition for mates in the ancestral environment (as it is among our primate cousins), tendency toward violence was adaptive for ancestral men but not for ancestral women. Accordingly, violent men have more sons, both in the United Kingdom and the United States, and, as I elaborate in an earlier post, this can potentially explain why many battered women may choose to stay with their abusive mates (so that they can produce violent and hence intrasexually competitive sons).
In my next post, I will discuss yet another factor which subtly influences the sex of the offspring: Parent’s physical attractiveness.
Why beautiful people have more daughters, and why women are more beautiful than men
Physical attractiveness can also bias the sex of your children. Now, unlike being big and tall or having a tendency toward violence, which increases the reproductive success of only men and not women, being beautiful is good for both men and women. Beautiful women have greater mating success than less attractive women, and handsome men do better than less attractive men. But beautiful men and beautiful women tend to do “better” in slightly different ways.
Physically attractive women tend to do well both in long-term and short-term mating; men prefer beautiful women for both. In contrast, handsome men tend to do well mostly in short-term mating. Women seek out handsome men for short-term mating (possibly to get good genes for their children by being impregnated by them but then passing the resulting offspring off as that of their unsuspecting husband) but not necessarily for long-term mating, for which other qualities like the man’s resources and status become more important. In fact, as I explain in a previous series of posts (Why handsome men make bad husbands Part I, II), physically attractive men may not make desirable long-term mates, precisely because other women seek them out for their short-term mating and thus attractive men are less committed to their long-term mates.
So physical attractiveness, while a universally positive quality, contributes even more to women’s reproductive success than to men’s. The generalized Trivers-Willard hypothesis would therefore predict that physically attractive parents should have more daughters than sons. Once again, this indeed appears to be the case. Young Americans who are rated “very attractive” have roughly a 44% chance of having a son for their first child (and thus a 56% chance of having a daughter). In contrast, everyone else has roughly a 52% chance of having a son (and thus a 48% chance of having a daughter) for their first child. Being “very attractive” increases the odds of having a daughter by 36% or decreases the odds of having a son by 26%!
The greater tendency of beautiful parents to have more daughters leads to another consequence. If you look around and rate the men and women around you on their physical attractiveness, you will notice that, whether you are a man or a woman, gay or straight, women on average are objectively more attractive than men. Why might this be the case?
Thinks about it. If physical attractiveness is heritable, such that beautiful parents beget beautiful children (and less attractive parents beget similarly less attractive children), and if beautiful parents are more likely to have daughters than others, then it logically follows that, over time, generation after generation, women will become more attractive on average than men. Once again, studies confirm this implication of the generalized Trivers-Willard hypothesis. The average level of physical attractiveness among women is significantly higher than the average level of physical attractiveness among men, both in Japan and the United States. Women are more beautiful than men on average because beautiful parents have significantly more daughters than less beautiful parents.
In my next post, the last one in this series, I will discuss another factor which seems to influence the offspring sex ratio: Parent’s sociosexual orientation.
Sexually promiscuous parents have more sons
The generalized Trivers-Willard hypothesis suggests that any heritable trait of a parent which is disproportionately beneficial to either sons or daughters can bias the sex of the offspring in the direction that benefits the parent’s reproductive success. It has so far been shown that engineers and scientists are more likely to have sons while nurses and social workers are more likely to have daughters; big and tall parents are more likely to have sons; violent parents are more likely to have sons; and beautiful parents are more likely to have daughters. Another factor which can subtly influence the sex of the offspring is the parent’s sociosexual orientation, how sexually promiscuous the parent is.
In our article forthcoming in the Annals of Human Biology, Péter Apari of Eötvös University and I extend the generalized Trivers-Willard hypothesis to sociosexual orientation. Compared to those with restricted sociosexual orientation, individuals with unrestricted sociosexual orientation are more likely to: 1) engage in sex at an earlier point in their relationships; 2) engage in sex with more than one partner at a time; and 3) be involved in sexual relationships characterized by less investment, commitment, love, and dependency. While men in general are more unrestricted in sociosexuality than women, within-sex variance in sociosexual orientation is much larger than between-sex variance.
Due to the sexual asymmetry in reproductive biology, unrestricted sociosexual orientation could potentially and dramatically increase men’s reproductive success, while it is likely to decrease women’s. Men with unrestricted sociosexual orientation can impregnate a large number of women simultaneously, and, even without male parental investment, some of the resultant children are likely to survive to sexual maturity. In contrast, women with unrestricted sociosexual orientations can have no more children than their restricted counterparts with one regular sex partner, and are unlikely successfully to secure male parental investment from the fathers of their children because none of the men can be reasonably sure of their paternity. Further, women in committed relationships who are sociosexually unrestricted may incur significant somatic costs of spousal and partner violence, because male sexual jealousy is often triggered by real or imagined sexual infidelity, and many mated men often use violence as a tactic for mate retention.
There is evidence to suggest that sociosexual orientation as well as risk of divorce, which often results from unrestricted sociosexual orientation, are highly heritable. A study of a large sample of Australian twins, conducted by the behavior geneticist J. Michael Bailey, whom we have encountered before, and colleagues, shows that nearly half of the variance in sociosexuality is attributable to genes. In other words, Madonnas beget Madonnas, and whores beget whores. As I explain in an earlier post, the risk of divorce is another heritable trait which follows the 50-0-50 rule: about 50% of the variance is heritable, about 0% is due to shared family environment, and about 50% is due to unshared environment. So roughly half of the variance in the risk of divorce, as well as sociosexual orientation, is heritable.
The logic of the generalized Trivers-Willard hypothesis would therefore suggest that individuals with unrestricted sociosexual orientation (“whores”) are more likely to have sons than those with restricted sociosexual orientation (“Madonnas”). Our analyses of two large nationally representative samples, from the General Social Survey in 1994 and the National Longitudinal Study of Adolescent Health, confirm this prediction. Controlling for a large number of social demographic factors that might be expected independently to influence offspring sex ratios, unrestricted sociosexual orientation significantly increases the odds that the first child is a boy. One standard deviation increase in the unrestrictedness of sociosexual orientation increases the odds of having a son by 12-19%.
It therefore appears that sociosexual orientation, whether you are relatively restricted or unrestricted in your sexual behavior, is yet another factor that might subtly and slightly influence the sex of your child, along with your brain types, body size, tendency toward violence, and physical attractiveness.
http://www.psychologytoday.com/blog/the-scientific-fundamentalist/200902/boy-or-girl-what-determines-the-sex-your-child-ii
http://www.psychologytoday.com/blog/the-scientific-fundamentalist/200902/boy-or-girl-what-determines-the-sex-your-child-i
http://www.psychologytoday.com/blog/the-scientific-fundamentalist/200902/boy-or-girl-what-determines-the-sex-your-child-iii
http://www.psychologytoday.com/blog/the-scientific-fundamentalist/200902/boy-or-girl-what-determines-the-sex-your-child-iv
Yet, I've read on many forums about a new phenomena in Europe, especially Eastern Europe, whereby brachcephals are increasingly having dolichocephal children. Why is this happening?
Consider this theory, outlined in the articles below. I know its a lot of text, but I can't succinct it better than the author does.
The genius of Robert L. Trivers
It is commonly believed that whether parents conceive a boy or a girl is entirely up to chance. Close enough, but not quite. It is largely up to chance, but there are factors that very subtly influence the sex of an offspring. It is also commonly believed that exactly half the babies born are boys and the other half are girls. Close enough, but not quite. The normal sex ratio at birth is .5122 – 105 boys for every 100 girls. But the sex ratio varies slightly in different circumstances and for different couples. So what factors affect the sex of the child?
Any discussion of sex ratio at birth must begin with the work of Robert L. Trivers, who is one of the greatest evolutionary biologists of his generation. In 1973, Trivers teamed up with a mathematician, Dan E. Willard, to formulate one of the most celebrated principles in evolutionary biology, called the Trivers-Willard hypothesis. The hypothesis states that wealthy parents of high status have more sons, while poor parents of low status have more daughters. This is because children generally inherit the wealth and social status of their parents. Sons from wealthy families, who themselves become wealthy, have, throughout most of evolutionary history, been able to expect to have a large number of wives, mistresses, and concubines, and produce dozens or hundreds of children, whereas their equally wealthy sisters can have only so many children. So wealthy parents should “bet” on sons rather than daughters.
Conversely, poor sons can expect to be completely excluded from the reproductive game, because no women would choose them as their mates. But their equally poor sisters can still expect to have some children if they are young and beautiful. The “fitness ceiling” – the best one can do reproductively – is much higher for men than for women, but the “fitness floor” – the worst one can do reproductively – is much lower for men. So poor parents should “bet” on daughters rather than sons. Natural selection designs parents to have a biased sex ratio at birth depending upon their economic circumstances – more boys if they are wealthy, more girls if they are poor.
There is evidence for this hypothesis throughout human societies. American Presidents, Vice Presidents, and cabinet secretaries have more sons than daughters (although only during the first half of its history). Poor Mukogodo herders in East Africa have more daughters than sons, both at birth and in the 0-4 age group. Church parish records from the 17th and 18th centuries in Germany show that wealthy landowners in Leezen, Schleswig-Holstein, had more sons than daughters, while farm laborers and tradesmen without property had more daughters than sons. Among the Cheyenne Indians on the American Plains, prestigious, high-status “peace chiefs” have more sons than daughters, while poor and marginal “war chiefs” have more daughters than sons in the 0-4 age group. In the contemporary United States and Germany, the elite – judged by the listing in their respective country’s Who’s Who – have a greater proportion of sons among their offspring than does the population in general. In an international survey of a large number of respondents from 46 different nations, more wealthy individuals are more likely to indicate a preference for sons if they could only have one child, whereas less wealthy individuals are more likely to indicate a preference for daughters. While there is some counterevidence, most evidence is in support of the Trivers-Willard hypothesis.
In my next post, I will discuss how Trivers’s genius in the Trivers-Willard hypothesis has been extended to new directions.
Extending Trivers’s genius
In my previous post, I introduce the Trivers-Willard hypothesis, which predicts that, relative to the baseline offspring sex ratio of 105 boys for 100 girls, wealthy families are more likely to have sons while poor families are more likely to have daughters. More recently, there has been a theoretical extension of the original Trivers-Willard hypothesis, called the generalized Trivers-Willard hypothesis.
The idea behind the new hypothesis is the same as that behind the old one, but it extends the idea to many other factors besides the family’s wealth and status. The new hypothesis suggests that if parents have any trait they can pass on to their children that is better for sons than for daughters, then they will have more boys. Conversely, if parents have any trait they can pass on to their children that is better for daughters than for sons, then they will have more girls. Parental wealth and status are just two of the traits they can pass on to their children that are more beneficial for sons than for daughters, but there are many other factors.
It is important to note that neither the original Trivers-Willard hypothesis nor the generalized Trivers-Willard hypothesis predicts that parents with a given set of heritable (and immutable) traits will produce offspring of only one sex or the other. Assume that the sex of a child is determined by a toss of an imaginary coin, and the “normal” or baseline parent has an “unbiased” coin that comes up “boy” 51.22% of the time and comes up “girl” 48.78% of the time (to reflect the “normal” baseline secondary sex ratio of 105 boys to 100 girls). If a parent possesses some heritable trait that is better for sons than for daughters, then the coin becomes “biased” in favor of sons and now comes up “boy,” say, 55% of the time. Such a parent is therefore significantly more likely to have a boy than the “normal” or baseline parent. However, it does not mean that such a parent will only have boys. It is still possible (albeit less likely than for others) for a parent with a probability of having a boy of .55 to have a girl, or even three girls in a row.
Brain types are a good example of heritable trait which affects boys and girls differently. Strong “male brains,” which are good at systemizing (figuring things out), are more beneficial for sons than for daughters, while strong “female brains,” which are good at empathizing (relating to people), are more beneficial for daughters than for sons. Since brain types are heritable, the generalized Trivers-Willard hypothesis would predict that parents with strong male brains, such as engineers, mathematicians, and scientists, are more likely to have sons, while those with strong female brains, such as nurses, social workers, and schoolteachers, are more likely to have daughters. This indeed appears to be the case. While the sex ratio at birth among the general population is .5122 – 105 boys for every 100 girls – the data show that the sex ratio among engineers and other systemizers is .5833, that is, 140 boys for every 100 girls. The comparable sex ratio among nurses and other empathizers is .4255, that is, 135 girls for every 100 boys.
By the same token, as I mention in a previous post, tall and big parents have more sons and produce more male fetuses (because body size was a distinct advantage in male competition for mates in the ancestral environment, while body size has no particular advantage for women), and short and small parents have more daughters and produce more female fetuses. (And, as I explain in another post, this observation can potentially explain why more boys are born during and after major wars.) Because violence was probably a routine means in the male competition for mates in the ancestral environment (as it is among our primate cousins), tendency toward violence was adaptive for ancestral men but not for ancestral women. Accordingly, violent men have more sons, both in the United Kingdom and the United States, and, as I elaborate in an earlier post, this can potentially explain why many battered women may choose to stay with their abusive mates (so that they can produce violent and hence intrasexually competitive sons).
In my next post, I will discuss yet another factor which subtly influences the sex of the offspring: Parent’s physical attractiveness.
Why beautiful people have more daughters, and why women are more beautiful than men
Physical attractiveness can also bias the sex of your children. Now, unlike being big and tall or having a tendency toward violence, which increases the reproductive success of only men and not women, being beautiful is good for both men and women. Beautiful women have greater mating success than less attractive women, and handsome men do better than less attractive men. But beautiful men and beautiful women tend to do “better” in slightly different ways.
Physically attractive women tend to do well both in long-term and short-term mating; men prefer beautiful women for both. In contrast, handsome men tend to do well mostly in short-term mating. Women seek out handsome men for short-term mating (possibly to get good genes for their children by being impregnated by them but then passing the resulting offspring off as that of their unsuspecting husband) but not necessarily for long-term mating, for which other qualities like the man’s resources and status become more important. In fact, as I explain in a previous series of posts (Why handsome men make bad husbands Part I, II), physically attractive men may not make desirable long-term mates, precisely because other women seek them out for their short-term mating and thus attractive men are less committed to their long-term mates.
So physical attractiveness, while a universally positive quality, contributes even more to women’s reproductive success than to men’s. The generalized Trivers-Willard hypothesis would therefore predict that physically attractive parents should have more daughters than sons. Once again, this indeed appears to be the case. Young Americans who are rated “very attractive” have roughly a 44% chance of having a son for their first child (and thus a 56% chance of having a daughter). In contrast, everyone else has roughly a 52% chance of having a son (and thus a 48% chance of having a daughter) for their first child. Being “very attractive” increases the odds of having a daughter by 36% or decreases the odds of having a son by 26%!
The greater tendency of beautiful parents to have more daughters leads to another consequence. If you look around and rate the men and women around you on their physical attractiveness, you will notice that, whether you are a man or a woman, gay or straight, women on average are objectively more attractive than men. Why might this be the case?
Thinks about it. If physical attractiveness is heritable, such that beautiful parents beget beautiful children (and less attractive parents beget similarly less attractive children), and if beautiful parents are more likely to have daughters than others, then it logically follows that, over time, generation after generation, women will become more attractive on average than men. Once again, studies confirm this implication of the generalized Trivers-Willard hypothesis. The average level of physical attractiveness among women is significantly higher than the average level of physical attractiveness among men, both in Japan and the United States. Women are more beautiful than men on average because beautiful parents have significantly more daughters than less beautiful parents.
In my next post, the last one in this series, I will discuss another factor which seems to influence the offspring sex ratio: Parent’s sociosexual orientation.
Sexually promiscuous parents have more sons
The generalized Trivers-Willard hypothesis suggests that any heritable trait of a parent which is disproportionately beneficial to either sons or daughters can bias the sex of the offspring in the direction that benefits the parent’s reproductive success. It has so far been shown that engineers and scientists are more likely to have sons while nurses and social workers are more likely to have daughters; big and tall parents are more likely to have sons; violent parents are more likely to have sons; and beautiful parents are more likely to have daughters. Another factor which can subtly influence the sex of the offspring is the parent’s sociosexual orientation, how sexually promiscuous the parent is.
In our article forthcoming in the Annals of Human Biology, Péter Apari of Eötvös University and I extend the generalized Trivers-Willard hypothesis to sociosexual orientation. Compared to those with restricted sociosexual orientation, individuals with unrestricted sociosexual orientation are more likely to: 1) engage in sex at an earlier point in their relationships; 2) engage in sex with more than one partner at a time; and 3) be involved in sexual relationships characterized by less investment, commitment, love, and dependency. While men in general are more unrestricted in sociosexuality than women, within-sex variance in sociosexual orientation is much larger than between-sex variance.
Due to the sexual asymmetry in reproductive biology, unrestricted sociosexual orientation could potentially and dramatically increase men’s reproductive success, while it is likely to decrease women’s. Men with unrestricted sociosexual orientation can impregnate a large number of women simultaneously, and, even without male parental investment, some of the resultant children are likely to survive to sexual maturity. In contrast, women with unrestricted sociosexual orientations can have no more children than their restricted counterparts with one regular sex partner, and are unlikely successfully to secure male parental investment from the fathers of their children because none of the men can be reasonably sure of their paternity. Further, women in committed relationships who are sociosexually unrestricted may incur significant somatic costs of spousal and partner violence, because male sexual jealousy is often triggered by real or imagined sexual infidelity, and many mated men often use violence as a tactic for mate retention.
There is evidence to suggest that sociosexual orientation as well as risk of divorce, which often results from unrestricted sociosexual orientation, are highly heritable. A study of a large sample of Australian twins, conducted by the behavior geneticist J. Michael Bailey, whom we have encountered before, and colleagues, shows that nearly half of the variance in sociosexuality is attributable to genes. In other words, Madonnas beget Madonnas, and whores beget whores. As I explain in an earlier post, the risk of divorce is another heritable trait which follows the 50-0-50 rule: about 50% of the variance is heritable, about 0% is due to shared family environment, and about 50% is due to unshared environment. So roughly half of the variance in the risk of divorce, as well as sociosexual orientation, is heritable.
The logic of the generalized Trivers-Willard hypothesis would therefore suggest that individuals with unrestricted sociosexual orientation (“whores”) are more likely to have sons than those with restricted sociosexual orientation (“Madonnas”). Our analyses of two large nationally representative samples, from the General Social Survey in 1994 and the National Longitudinal Study of Adolescent Health, confirm this prediction. Controlling for a large number of social demographic factors that might be expected independently to influence offspring sex ratios, unrestricted sociosexual orientation significantly increases the odds that the first child is a boy. One standard deviation increase in the unrestrictedness of sociosexual orientation increases the odds of having a son by 12-19%.
It therefore appears that sociosexual orientation, whether you are relatively restricted or unrestricted in your sexual behavior, is yet another factor that might subtly and slightly influence the sex of your child, along with your brain types, body size, tendency toward violence, and physical attractiveness.
http://www.psychologytoday.com/blog/the-scientific-fundamentalist/200902/boy-or-girl-what-determines-the-sex-your-child-ii
http://www.psychologytoday.com/blog/the-scientific-fundamentalist/200902/boy-or-girl-what-determines-the-sex-your-child-i
http://www.psychologytoday.com/blog/the-scientific-fundamentalist/200902/boy-or-girl-what-determines-the-sex-your-child-iii
http://www.psychologytoday.com/blog/the-scientific-fundamentalist/200902/boy-or-girl-what-determines-the-sex-your-child-iv