It is general regarded as a solid fact that women reach full maturity earlier than men, and this almost certainly applies to brains - and therefore almost certainly applies to intelligence - yet this fact is very generally neglected.
(Except by Richard Lynn - whose argument is apparently being ignored - although surely it is far more biologically plausible than the alternative tacit assumption that a 16 year old man and women are at exactly the same stage of development.)
Suppose, for the sake of discussion, that on average women reach their maximum intelligence at 16 and men at 18 - the actual numbers are not really known, because it seems that the matter has not been thoroughly looked-into.
The average 16 year old woman is at her maximum intelligence, while the average sixteen year old man is at about 16/18ths of his maximum intelligence - that is to say the average 16 year old man is at somewhat less than 90 percent of his full intelligence (assuming that development is linear).
To get a more precise estimate of the maturation of men and women - probably the best measurement would be serial simple reaction times (sRTs) - sRT could be measured every few months in a longitudinal study of individual children right through into their twenties - and the (average) age at which the sRT reaches its shortest (i.e. the average age when reaction times are fastest) would be the age of full cognitive maturation for intelligence.
The implications of this matter are considerable.
1. When comparing boys and girls or young men and young women - they should not be age-matched, but maturation matched - so that an 8 year old girl who is at about 50 percent maturity, should be matched with a 9 year old boy - who is at an equivalent level of maturity - and so on.
2. What does the age difference in maximum intelligence mean - in biological terms?
It seems plausible that intelligence is a consequence of brain complexity - in the sense that as a general principle (from systems theory, and confirmed by biology) increased complexity of organization allows (and it necessary for) increased efficiency - and general intelligence is substantially about greater brain efficiency.
But increased complexity of brain organization implies:
i) A longer period of brain development
ii) A greater potential for things to go wrong during brain development.
So, on these grounds we would expect the 'more-intelligent' sex (i.e. men) to have on the one hand a longer 'normal' (which is not necessarily the same as 'average') period of brain development with later maturation (which is indeed observed);
and, on the other hand, also a higher rate of things going wrong and leading to sub-optimal intelligence (which is also observed - in that there are more men than women who suffer from mental handicap/ subnormal intelligence).
Also, it would be assumed that - in considering genetically-separated populations, the 'normal' period of brain development would have a broad correlation with average intelligence in that population (assuming other relevant influencing factors were sufficiently controlled) - so that the most intelligent populations had a significantly longer development than the least.
This also seems to be the case - and the shortest normal development period - which is, I think, seen in pygmies who mature at about age 12, seems to be associated with low average intelligence, and so on.
In passing, it may be suggested that the reducing average age of sexual maturation that has been seen in Western populations may also be happening at the same time as reducing average intelligence in the same populations - and this statistical association may (to some significant extent - although not necessarily entirely) be causal.
One interesting conclusion of this line of research is that when young boys and girls of the same age test at the same level of intelligence -
this is actually strong evidence that adult men are more intelligent than adult women!
Another implication of this line of reasoning is that we should be less interested by differences (or not) between averages when comparing men and women or different populations, and more interested by differences at extreme or peak levels.
In other words, we should think of intelligence more like we think of athletic ability - when we are interested in the best, rather than the average.
For example, the interesting men-women comparison is between (say) Olympic standard runners and weight lifters, or professional tennis or soccer players - rather than between the population average ability - which is mostly unknown and also very difficult to measure (due to the in-practice impossibility of getting truly random population samples).