The curve of mortality versus age has a broad minimum at about the age of 12.  It climbs steeply beyond middle age.

Age 12 is close to the threshold for sexual activity.  It is almost the age when parental investments begin to pay off.  This must not be a coincidence.  It has been speculated that genes that provide for a low mortality at this age have a greater benefit than genes affecting mortality at any other age.

The steep rise after middle age has another interesting story behind it.  If we could produce a plot versus age of the influence of a typical ancestor upon the fate of his genes, the plot would have a peak occurring at an age somewhere in the twenties.  At this age children are being produced and raised.  Parental investment rate is near its maximum.  During the 30s parental investment begins to shift toward child raising instead of births.  During the 40s births are replaced with the raising of children and the beginning of grand-parent investing.  During the 50s raising is over, and only grand-parenting remains.

As the impact of a person upon his progeny declines, so does the importance of protecting the person from diseases and other health problems.  Genes that protect the 50-year old would have a more difficult time being "selected for" by the forces of evolution than genes that protect the 20-year old.  In fact, since every gene carries with it a burden of unintended disturbances of things done by other genes, there actually might be merit in NOT selecting for the 50-year old health protecting gene.

There's a good part to this story, however.  After middle age there is little need to keep blinders on the individual.  If he wants to liberate himself from the power of the genes, and create a new agenda for the remainder of life, there is little incentive for the genes to create inhibiting brain circuits to prevent this from happening.

It's as if an individual is free to do what he wants after he has done the work of his genes.  Freedom must be earned.

A person in his 20s who desires freedom must contend with brain circuits that will pressure him to return to the genetic script if he tries to depart from it.  A man in his 50s must contend only with the impulse to be a good grandfather and make young women pregnant, which is a lighter burden than providing parental investments.  The genes have less at stake for such a man, so it should be easier for him to walk away from his instincts than it is for a man in his 20s.  Women in their 50s, who want to create their own lifestyle, must contend with the instinctual desire to help with grandchildren.  This must be an easier force to overcome than the procreational forces that existed in her 20s.  Thus, both men and women should find it easier to define their lives after middle age.

I met a sociobiology graduate student at a sociobiology symposium a few years ago.  When I told him I wanted to write a "sociobiological philosophy," and I sketched some of what it might contain, he seemed surprised and interested in the idea.

I said to him "For example, you're in your 20s now, aren't you?"  "Yes."  "Well, I'm in my 40s, and I can shape a new lifestyle much more easily than you can.  You can try as hard as you want, but you will fail.  It'd be like trying to climb a steep wall!  But for me, I only have to walk up a slightly sloped incline."

"I have felt the grip of the genes, like the butterfly that has been captured inside protective hands.  The genes are almost through with me, and they are releasing their grip.  As the hands open, I see daylight.  My time to escape is almost here, and I am ready to fly away.  You are still in the grip of your genes, and you must wait a couple more decades before you can expect to see this daylight."

Just then, the coffee break ended, and we were called back to the auditorium.  I wonder if he remembers our conversation.

This site opened:  November 17, 1998.   Last Update: November 17, 1998