Geneticists say that the level of mental development of the children born of parents who are cousins is expected to be lower than that of "ordinary" children. This fact is confirmed, in particular, by studies conducted in Japan, where there are a lot of marriages between close relatives. Scientists conclude that the mental development is conditioned genetically by approximately 80 percent and only by twenty percent - environmentally. So what then may be considered the "intelligence criterion"?

Some specialists suppose that an indicator of this criterion is the speed of mental processes, the speed of solving intellectual problems. It has been found that those who quickly solve easy problems, also need less time than others to solve difficult ones, and those who solve easy problems slowly, take more time than others for difficult ones. At one time it was demonstrated that with the increase of the difficulty of a problem the time for its solution is increased logarithmically. It means that the correct solution of intellectual problems of a certain complexity may require from some people days, weeks, months, and even years.

Of course, a great role here is played by such psychological characteristics as perseverance and concentration.

The studies have also shown that men and women have an approximately identical level of mental development, though there are also some differences. Men, as a rule, better manage numerical and dimensional problems, women are better in solving verbal tasks and tests on memorizing. The range of the "intelligence coefficient" variation for men is greater that for women: there are more very "clever" and very "stupid" men than women belonging to these extreme categories, however, "clever" women are found more frequently than "clever" men. The reasons for such differences are not yet known. Besides, the studies of the history of the mankind intelligence development from historical times up to now have shown no evidence of any change in human intelligence for the last three thousand years. Possibly, the ancient Greeks had the same capacities as modern people. And, who knows, they may have been even higher!

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