Monday, 17 February 2014

How does high intelligence evolve?


The simplest way is when (on average) only those of above-average intelligence are able to raise children to sexual maturity.


The assumption is that (in pre-industrial society) almost everybody has above-replacement fertility (averaging significantly more than two children per women) but a situation in which people of lower intelligence almost-never raise any children to adulthood because almost-all children die before reaching sexual maturity.


This was, in fact, probably the situation that prevailed in Medieval Britain, and probably Western Europe generally, and probably pre-modern China and East Asia, and among Ashkenazi Jews in the Middle Ages.


You can think of this as a bottom-threshold, or as (barring rare flukes) a minimum intelligence level for rearing children to adulthood...

And this minimum intelligence necessary to rear children to adulthood being at above average intelligence - and this situation prevailing as a selection environment for long enough to raise the average level.

Of course, what will most likely happen is that as intelligence declines, there is a sharply-declining-probability of raising children to adulthood - a probability which reaches near-zero at somewhere around average intelligence for that group.


The point is that there is not much need for evolving genes associated with higher intelligence, rather the mechanisms is mostly one of a selecting-out of genes associated with low intelligence.

These selected-out genes may have other, not-intelligence-related advantages, which would, of course, be lost.

Possible/ plausible examples of selected-out genes (from surveying the higher intelligence populations) are genes for athleticism (e.g. genes associated with better running, jumping, one-on-one unarmed combat).


And of course this selection process for higher intelligence pretty-much ceased to operate in developed countries from about 1800-1850 - when childhood mortality rates began to plummet towards zero among the least intelligent; and selection became (more or less) for 'pure fertility' - so that any genes associated with any behavioural cause of maintained/ increased fertility (including genes which damage biological functions, and render someone unable/ unwilling to use fertility controlling technologies) would be, have been, amplified in this post-industrial modern population.  


James Purcell said...

Do you think the rise of diabetes and other chronic diseases are related to dysgenic fertility?

Bruce Charlton said...

Not diabetes - but probably other things.