There are many pseudopsychological books attempting to explain the relationship of intellectual abilities of the child from his birth order in the family. All they greatly overestimate the link between these two phenomena. However, the fact that such a relationship exists, there is no doubt about it. The first-born in a family, as a rule, before beginning to speak, more active, do better in school and often show higher on tests of intelligence. Figure out how to actually the case with birth order and its effect on the child's intelligence, the researchers succeeded in the 70-ies of XX century.
In the early seventies in the Netherlands carried out a major research project. It was supposed to study the effect of malnutrition on intellectual ability of children born at the end of the Second World War. Part of this research was test Raven's intellectual development: it performed 350,000 Dutch men aged 19 years. Two researchers working on this project, quite unexpectedly saw a close relationship between birth order of the subject and its test results: the more the subject of older brothers or sisters that were below his performance on the test Raven.
Interpretation of these data gave other scientists — Robert Zajonc and Gregory Markus. They suggested that children have a higher intellectual capacity, when growing up in an environment that stimulates their development. Scientists have proposed to introduce such a measure, the average IQ of the family. When a couple have a child, intellectual family environment is created by two adults and one infant. If assume that every adult has a conditional rating of 100 points (this is not IQ, and arbitrary scale), the newborn — to 0 points. Therefore, to calculate the average intellectual the level of the family, it is necessary to lay down the intellectual contributions of all members of the family and divide by the number of them. In a family with a newborn it would be 67 (ie, 100 + 100 + 0/3 = 67). Now suppose that the contribution of the child in the intellectual climate of the family each year increased by 5 points. If the family after 2 years will be born a second baby, then the general intellectual level of the family will already 52.5 points.
But does this mean that the more children, the more "tupeet" family? If you create a model in which every two years in the family will emerge a new baby, it becomes clear that the intellectual level of the family falls before the birth of the fifth child, and then slowly begins to go up.
However, the education of the five (or more) child is ready, not every family. But does this dynamic intellectual development, the fewer children in the family, the more it affects their mental abilities. As it turned out, not quite. In the Dutch study, said so-called "effect of an only child." Estimates only children under the test Raven on average were on the same level with the results of the older children in families with four children.