It stabilizes about age 6, but infant-young child intelligence scales correlate poorly with intelligence measured later. Note why this tends to be so. Potent environmental influences on measured intelligence are well known. The method of kinship correlations Correlate the IQs of pairs of individuals with varying degrees of relationship. We look for correlations increasing with closer relationship.
Correlations increase with biological relationship, even when pair members are reared apart.
The correlations were respectively .67, .77, and .62. Often, the separation did not occur early, and the environments of the separated twins were very similar. Newer studies are attempting to answer these critical questions. The results of these studies do not suggest high heritability. In this review of studies on the possible heritability of intelligence, we are talking about one theoretical view of intelligence, the notion of general intelligence.