Declining teen sex:
I suppose everybody agrees that teenagers are often very good looking and that they have enormous vitality.  They have the lowest overall death rate of any age group.

In spite of this it is not so safe for them to have babies as it becomes a few years later.  The girls’ bodies simply are not yet sufficiently mature.  So it is a good thing that the number of teen pregnancies is falling.  So far we agree.  And in fact a study done by the CDC in Atlanta has evidence that this is the case.  Those who comment, such as I, are generally very pleased.

But there may be more to it.

The study done by the CDC (sorry I don’t have the reference) interviewed some five thousand people between ages 15 and 24.  They did this in 2002 and again from 2006 through 2008.  Let’s say that is a period of 4 years difference between the times the studies were started.  Over that time the number reporting they had had no sex ever rose from 22% to 28%, a rise of about 1.5% per year. 

We used to be told that a person who had not had sex by the age of 24 was by and large never going to have sex.  I do not know if that continues to be the belief.  But let us assume it in order to get some numbers.  And also let as assume that the average person interviewed was 20 years old.  If 72% of people in this country are still having sex at that age, then the number falls to zero in about 67 years.  That means the last person who will have sex will be born some 47 years from now.

Of course the number could rise faster or slower.  It depends on what the cause is.

Others who comment say the change is due to prudence.  I am of the opinion that in matters reproductive people are very emotional.  Talk to people about sports, about their government, their country, their religion, their car and you may find that by and large everyone is an emotional basket case.  Are we then to assume right off the bat that when it comes to sex people are cool headed, rational, steady of nerve and foresighted of intent?  That is hard to believe.

So why should our emotions be changing in this regard? 

I did not predict it, but I should have.  We have been so industrious in avoiding cousin marriages that we are probably beyond recall by ordinary means.  Nature has decided that we are doomed and that the thing to do is to take us out of the picture.  That is because we are a threat to the entire species.

As I pointed out in the Vancouver poster, nature takes pains to limit population sizes on account of long term speciation issues.  Usually that works quite well.  It has worked for us for a long time, with population after population dying out.  But evolution never had to cope with global travel before, or even bicycles.  Everyone now has the opportunity to marry outside the viable circle and just about everybody is taking that opportunity. 

So having us stop having sex is part of Mother Nature’s plan to rescue us from our own folly.

Thanks, Ma.  It’s not working. 

The reason I should have predicted it is because there are other similar things going on.

  1. Fertility is falling or has already dropped below survivable levels everywhere.  Check out
  2. Where fertility has fallen below replacement levels, by and large women are marrying later and later.  Check gapminder again.
  3. Women may have smaller pelvic outlets.  The only evidence I have for that is that there was a brief flap some time ago because of increasing maternal deaths.  They managed to run down the fact that the increase was due to an increase in the number of caesarian childbirths.  Almost invariably that procedure is done because of a small pelvic outlet.  However the way of measuring pelvic outlets has changed so we probably won’t get confirmation on this. 
  4. Male genital development is declining.  I have seen that in a good source I cannot call up.
  5. Hypospadias is increasing.  (That is form of slightly feminized male development.) 
  6. Prejudice against marrying cousins is increasing.
  7. Nobody seems to care.


Any one of those is or at least can be driven by inherited effects.  And any one of them if taken to an extreme would wipe us off the planet.

So if fewer people are having sex, it’s part of a larger pattern.  Time will tell, of course, but if you don’t like it, it is time to act now or maybe it was time to act a long time ago. 

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