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Wealthy mums 'have more sons'

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  • Wealthy mums 'have more sons'

    Wealthy mothers have more sons while their less privileged counterparts tend to produce more daughters, a study shows.

    08 Jul 2009

    Women are tougher than men so evolutionary theories suggest those living in poorer communities are predisposed to giving birth to girls to ensure survival of the family line - because the men are more at risk of dying younger.

    New research has now borne out the idea by analysing a large database of over 95,000 Rwandan mothers in either monogamous unions involving a single couple or polygynous marriages - a form of polygamy, where the man has several wives.

    The low-ranking wives in polygynous marriages should be in worse condition because they get a smaller share of their husband's resources.

    And as expected these women produced more daughters than higher-ranking and monogamous wives, according to the findings published in Biology Letters.

    Social psychologist Dr Thomas Pollet and colleagues at Groningen University in the Netherlands said this reveals a "previously undetected form of sex-ratio biasing in humans".

    The biggest difference was between third or lower-ranking wives who had 106 daughters for every 100 sons compared with those in monogamous marriages who had 99 daughters for every 100 sons.

    Dr Pollet said: "Our findings show that low-ranking wives, of third order or lower, have lower fertility than other women, suggesting that they are in poorer condition.

    "These low-ranking wives have relatively more daughters than higher-ranking and monogamously married wives.

    "This fits with previous findings from non-Western cultures documenting female-biased sex ratios as a function of maternal condition.

    "Mothers in poor condition, here lower-ranking co-wives in a polygynous marriage, may overproduce daughters because these give them greater fitness returns than sons."

    In America for instance, black mothers have long born fewer boys than white mothers who have seven more for every thousand births.

    And it has a depressing implication for countries such as India and China where female infanticide occurs since the findings suggest these countries cannot avoid having an excess of daughters.

    The idea has proved true for a range of species such as insects, birds, pigs, sheep, dogs, mink and deer.

    Previous research has also shown married, better educated and younger mothers are more likely to bear sons.

    Three decades ago Robert Trivers, an evolutionary biologist, and Dan Willard, a mathematician, boiled life down to one factor - having as many offspring as possible and so passing on as many genes to future generations as possible.

    The "Trivers-Willard hypothesis" argues strong healthy women tend to have sons in order to ensure her genes and family line are passed on.

    These sons are in turn strong and outdo other weaker male offspring to reproduce - thus ensuring survival of the fittest and of the family line.

    But weaker or poor mothers tend to have weaker sons who do not do well and are more likely to die early - earlier than women in the same society.

    To ensure survival of the family line, the women tend to have daughters because they are more likely to be survive long enough to become parents themselves.

    Dr Pollet said: "There is considerable debate on whether human females bias the sex ratio of their offspring as a function of their own condition.

    "We apply the Trivers-Willard prediction - that mothers in poor condition will overproduce daughters - to a novel measure of condition, namely wife rank within a polygynous marriage.

    "Using a large-scale sample of over 95,000 Rwandan mothers, we show that lower-ranking polygynous wives do indeed have significantly more daughters than higher-ranking polygynous wives and monogamously married women.

    "Our findings could also be explained by the maternal dominance hypothesis which states dominant women are more likely to conceive sons because they have higher circulating levels of testosterone.

    "Marriage rank could be related to testosterone levels in our study population, but we lack the data to investigate this. However, this hypothesis is a proximate explanation and is perfectly compatible with the ultimate explanation we have focused on here."
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  • #2
    Re: Wealthy mums 'have more sons'

    More girls born in tropical countries

    More girls are born in tropical countries than areas with cooler climates.
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