May 27, 2002
8:10 AM   Subscribe

Mothers who wait to have a baby are at risk of evolutionary extinction. "If you want to see your line persist, then it's probably optimum to start reproducing in your early to mid-20s". According to this 220 year statistical model late-reproducing women [genetic lineage] declined as a proportion of the population from 11 percent to about 5 percent
posted by stbalbach (8 comments total)
There are so many things wrong with this study. Hmm, where do I begin?

1. A late-reproducing lineage will evolve better late-fertility very quickly and members of this lineage will live much longer (this has been shown emperically in late-reproducing flies whose life span is double the wild-type lifespan)

2. Cultural norms, technology, the very definition of 'quality of life' changes dramatically over periods of 100's of years and will govern the decisions made by each generation about early or late reproduction.

3. Late reproducing moms will make fewer babies per mom because of a reduced reproductive period. Therefore, if you hold cultural factors (and life span) constant, it is a foregone conclusion that their progeny will make up an increasingly smaller proportion of society because Darwinian-type selection is no longer operative in modern-western society.
posted by plaino at 8:35 AM on May 27, 2002

"In any species, other things being equal, whoever keeps their family line going and growing, persists while others go extinct," explains Low,
Once you read this you discount anything else that the author has to say. What it means: once you don't continue your family line you can not continue your family line. Insight!
posted by Postroad at 8:54 AM on May 27, 2002

I choose D: as all of the above said.

how about if we outlaw any male or female on the planet from reproducing before the age of say, fourty. The ones that can't reproduce that late in age, die out, the ones that can may actually be helping all humankind live longer in general (theoretically it could increase our lifespan).
posted by dabitch at 9:09 AM on May 27, 2002

I'll volounteer for the reproduction process !

Jokes aside, maybe the reason behind the late-age reproduction are simple as:

1) it requires money to raise kids
2) it takes time to raise kids

and both are in scarse supply, but that's old news.
Expecially time is important in this equation:

+-time +- money +- will to have a kid = # kids born from a couple

Expecially consultants, professionals, or people that do invest a lot of their time into their job because of the omnipotent needs of "market" are likely to have a kid
later in their life, or as soon as they have enough money to invest less time in career.

People with less pressure on the time factor often have to deal with the money factor , I'm speaking about people
that work from hour X to Y and have little chances of getting a better job, or that are happy with it. They probably are more likely to have a kid sooner then later in their life, probably only one or at best two.

Of course will to have a kid can compensate the lack of
money and time, no matter how dangerous the lack is.
See for example people with 3-4 kids and with only
one source of income. Mother or father compensates the
time-need by not working , or by giving up chances of
career. The other parent provides financial support.

Often relatives and grandparents / relatives are kept out of equation, but they really are important because they can provide either time or money.

Last but not least, people want to have more fun. There's a ton of entertainment being pushed by media / travel agents / theme parks etc etc. That reduces the time factor
because , you know, you just aren't completely "free" when
traveling with a kid.

So it seems to me that the results of the U-M analysis adds two factor
a) lineage success ??
b) death of a parent

While b) can be considered both from money and time point of view, how do we add a weight to lineage success ? And what's their definition of lineage success after all ? I don't care if my DNA will be here in the next 1001 years because of the number of my kids reproducing around the planet. That sounds so proto-eugenetics to me.
posted by elpapacito at 9:30 AM on May 27, 2002

I'm sure this study contains some interesting numbers, but its human application is irritatingly simple-minded. Does anyone actually care about their genetic lineage?

posted by Mars Saxman at 9:59 AM on May 27, 2002

also, in this study did they assume that children of late-bearing moms will also be late bearing moms, and children of young mothers will follow that decision? i.e. that age of reproduction is genetically inherited somehow? Don't really see the logic there. "studies" based on simulations seem a little precarious to me.
posted by mdn at 10:14 AM on May 27, 2002

To add to all the above comments:
--Social class is not biologically inherited, though this is something that the study seems bizarrely to assume.
--A mistake about intentionality. No animals, and almost no human beings, are motivated by the conscious desire to have as many progeny as possible. In darwinian theory, behaviors that have this effect are selected for; but this should not be confused with it being a direct motivation.
--Just to go briefly over the whole science/culture argument. This is not disinterested, objective research. Even aside from the ridiculous assumptions made by the researchers, like the idea that the proclivity to have children later rather than earlier is somehow genetically controlled, it is not an arbitrary or innocent decision to discuss this particular issue--women in the workplace having children later in life--rather than some other research focus, like, say, the effect of poverty on children's chances later in life.
posted by Rebis at 10:16 AM on May 27, 2002

Darwinian-type selection is no longer operative in modern-western society. [plaino]

Not sure that's true. It's certainly being influenced by social factors that aren't strictly biologically necessary -- but that's always been true, and not just for humans. (One word: plumage.) At most, I'd say that we've added an economic variable to evolution, since people will select mates based on financial and social success as well as on who has the prettiest tailfeathers -- that doesn't eliminate evolution, it just changes it.

On the other hand, economies and cultures don't last on an evolutionary timescale. (Not so far, anyway.) So their effects, in the long-term, might be negligible. Hard to tell, unless you've got a few million years to spare. Interesting to think about, though.

Yes, Postroad, it may be stating the obvious to say that those who breed later will have fewer children -- but that's only the starting point of the study: if the carriers of particular genetic traits are tending to breed later (for social, not genetic reasons), then those traits are more likely to disappear.

Rebis, I think you've got it backwards: I don't see anything in this article that implies that they're arguing that the social factors are "somehow genetically controlled" -- looks to me the other way around, that they're studying how social factors may influence genetic evolution.

Personally I find this a very interesting, if imperfect, study; I'm not sure why it's receiving such unilateral opposition here. If you accept that the human race is continuing to evolve, one way or another, I think it's valid to explore in what ways cultural or social preferences are going to influence that evolution. Social sciences are tricky -- it's really difficult to isolate your variables, and there are indeed some assumptions in this study that seem questionable to me: mdn's point is a good one, for example (while it's possible that the preference for later or earlier childbearing would be passed down for purely cultural reasons, and therefore would have an effect on evolution without being a true inheritable trait, I doubt those cultural preferences would be stable enough to have more than a butterfly-wing effect.) Another statement of theirs I wonder about is that the "number of kids is no longer the name of the game" -- I don't follow that one at all; no amount of cultural influence can change the fact that more surviving kids == more surviving genes.
posted by ook at 4:15 PM on May 28, 2002

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