Differential reproduction for men and women.

7 comments and no answers...? Regardless, you could have answered this question pretty easily and I don't think this was Discussion-post-worthy (certainly a reasonable Open Thread question). But I'll answer your question anyway.

The second line of the linked talk says:

For more information on this topic, read Dr. Baumeister's book Is There Anything Good About Men? available in bookstores everywhere, including here.

A search of 'Is There Anything Good About Men' in the usual place turns up a copy. Download. What are we looking for? A reminder, the key lines in the linked speech are:

...It’s not a trick question, and it’s not 50%. True, about half the people who ever lived were women, but that’s not the question. We’re asking about all the people who ever lived who have a descendant living today. Or, put another way, yes,every baby has both a mother and a father, but some of those parents had multiple children. Recent research using DNA analysis answered this question about two years ago. Today’s human population is descended from twice as many women as men. I think this difference is the single most under-appreciated fact about gender. To get that kind of difference, you had to have something like, throughout the entire history of the human race, maybe 80% of women but only 40% of men reproduced.

We could search for various words or phrase from this passages which seem to be relatively unique; as it happens, I chose the rhetorical "50%" (but "80%", "40%", "underappreciated", etc all would've worked with varying levels of efficiency since the speech is heavily based on the book), and thus jumped straight to chapter 4, "The Most Underappreciated Fact About Men". A glance tells us that Baumeister is discussing exactly this topic of reproductive differentials, so we read on and a few pages later, on page 63, we hit the jackpot:

The correct answer has recently begun to emerge from DNA studies, notably those by Jason Wilder and his colleagues. They concluded that among the ancestors of today’s human population, women outnumbered men about two to one. Two to one! In percentage terms, then, humanity’s ancestors were about 67% female and 33% male.

A C-f for "Wilder" takes us to pg286, where we immediately read:

...The DNA studies on how today's human population is descended from twice as many women as men have been the most requested sources from my earlier talks on this. The work is by Jason Wilder and his colleagues. I list here some sources in the mass media, which may be more accessible to laypersons than the highly technical journal articles, but for the specialists I list those also.

For a highly readable introduction, you can Google the article "Ancient Man Spread the Love Around," which was published September, 20, 2004 and is still available (last I checked) online. There were plenty of other stories in the media at about this time, when the research findings first came out. In "Medical News Today," (www.medicalnewstoday. com), on the same date in 2004, a story under "Genes expose secrets of sex on the side" covered much the same material.

If you want the original sources, read Wilder, J. A., Mobasher, Z., & Hammer, M. F. (2004). "Genetic evidence for unequal effective population sizes of human females and males". Molecular Biology and Evolution, 21, 2047-2057. If that went down well, you might try Wilder, J. A., Kingan, S. B., Mobasher, Z., Pilkington, M. M., & Hammer, M. F. (2004). "Global patterns of human mitochondrial DNA and Y-chromosome structure are not influenced by higher migration rates of females versus males". Nature Genetics, 36, 1122-1125. That one was over my head, I admit. A more readable source on these is Shriver, M. D. (2005), "Female migration rate might not be greater than male rate". European Journal of Human Genetics, 13, 131-132. Shriver raises another intriguing hypothesis that could have contributed to the greater preponderance of females in our ancestors: Because couples mate such that the man is older, the generational intervals are smaller for females (i.e., baby's age is closer to mother's than to father's). As for the 90% to 20% differential in other species, that I believe is standard information in biology, which I first heard in one of the lectures on testosterone by the late James Dabbs, whose book Heroes, Rogues, and Lovers remains an authoritative source on the topic.

(I jailbroke Shriver 2005 for you. Wilder et al 2004, incidentally, fits well with Baumeister remarking in 2007 that the research was done 2 or so years ago.)

And of course you could've done the exact same thing using Google Books: search "baumeister anything good about men" to get to the book, then search-within-the-book for "50%", jump to page 53, read to page 63, do a second search-within-the-book for "Wilder" and the second hit of page 287 even gives you the exact snippet you need:

Sources and References 287

...If you want the original sources, read Wilder, J. A., Mobasher, Z., & Hammer, M. F. (2004). "Genetic evidence for unequal effective population sizes of human females and males". Molecular Biology and Evolution...

I've actually read more details about this. Apparently, it comes from studies of Y-chromosome DNA and mitochondrial DNA that indicate that the female common ancestor of all living humans is a lot older than the male common ancestor of all living humans. However, this past year, researchers discovered a man with a Y-chromosome that threw off the timeline drastically - which throws the conclusion of sex-based differential reproduction into question.

I imagine you could look at variations in mitochondrial DNA and Y chromosones to find out to some degree. No idea what that data shows though. IT sounds like the original author is trying to make a social point not a factual one about evolution.

EDIT Also, for most of history wouldn't the majority of both men and women never reproduce at all as they died before reaching reproductive age?

The article was written in 2007 and it says: "Recent research using DNA analysis answered this question about two years ago. Today’s human population is descended from twice as many women as men." So the research, if it exists, was done in 2005, plus or minus one year.

If we want to talk polygamy, we want to talk about African and Islamic cultures. But even where it is most accepted, actual polygamy rates are fairly low, though this is exaggerated by recent declines in Africa.

If there is something like a 80% to 40% difference - that would surprise me - I doubt that saying `polygamy' would cover it. If there is a big difference, it should be bigger, and I think a better and more familiar explanation would suit: men have relativity few time and resource constraints on reproduction. Looking at it historically, the costs of reproduction come overwhelmingly from pregnancy, birth, and risk of child mortality. Reproducing under (historically) typical conditions involves keeping women pregnant. So we should expect women to have a reproduction greater than or equal to that of males.

Start with 100 men and 100 women (generation 0). Now suppose that in each generation, 80% of adult men are successfully monogamous, a dominant 10% reproduce with two women, and 10% do not reproduce. To keep things simple, suppose each human pair produces an average of two successful children - one a man and one a woman - in that they live to be sorted into this 10/80/10 split. Each generation is assumed to die off to make math simpler for the kids, and I'm going to make a lot of stupid independence assumptions.

At generation 1, we have descendants for all 100 generation 1 women and for 90 of the generation 1 men.

At generation 2, we have descendants for all 100 generation 1 women and 81 of the generation 1 men have descendants.

At generation 3, we have descendants for all 100 generation 1 women and ~72 generation 1 men.

I think we can see where this is going. At generation n, we have 100*(9/10)^n men with descendants and 100 women, giving us a ratio of (9/10)^n men to women with descendants at generation n. I've also been assuming that children of dominant parents are as likely to be sorted into the dominant/monogamous/unsuccessful groups as any other child. I doubt that this assumption holds in reality, and think its failure would tend to reduce the ratio. [Edit: I neglected female-line descendants. This only holds for strictly male-line descendants. I'll work on something better.]

As for determining via experiment the actual reproduction rates for men and women throughout history, I am not optimistic. All the evidence we have points to relatively recent population bottlenecks. On a more obvious note, human populations over the last few centuries - and then millennia - are huge historical outliers. And within these outliers, there are catastrophic fluctuations due to plague and conflict, e.g. the black death or the Congo Free State. In the last few centuries especially, we've seen dramatic cultural shifts concerning marriage norms. Particularly relevant to polygamy would be the movement of people from mixed polygamous/monogamous/polygynous cultures to nominally monogamous ones.

Hm. This is also, kind of, a question about polygamy - maybe we could find the data that way. But then, isn't the monogamy vs polygamy debate still quite unsettled? If we just look at current societies, then according to wikipedia polygyny isn't near common enough to produce a 2:1 ratio. Or for example in the hunter-gatherer Maori tribes, the chiefs sometimes took two wives (one chief even had three), so logically another man didn't reproduce, but that's not even close to being 80% of women having kids with 40% of men.

Well I think what we have established is that it is all in DNA as most things are but what I ask is if there are more girls coming in families resulting in more ancestors then WHY are there more boys in the world

Whatever the true explanation, if you claim that the bell curves for male abilities are flatter than corresponding female ones (more extreme highs and lows for men), you are a terrorist.

Could it be an enhancement to change humanity into 65% women? or even more, thus giving on average more people a chance to parenthood?

Not to mention all the other reasons for more women, such as natural kindness, risk aversion, extended lifespan...