Thus this five-year program of intensive intervention beginning in early infancy increased IQ (at age fifteen years) by about five points. Judging from a comparable gain in scholastic achievement, the effect had broad transfer, suggesting that it probably raised the level of g to some extent. The finding that the T subjects did better than the C subjects on a battery of Piaget's tests of conservation, which reflect important stages in mental development, is further evidence. The Piagetian tests are not only very different in task demands from anything in the conventional IQ tests used in the conventional assessments, but are also highly g loaded.[ 57] The mean T-C difference on the Piagetian conservation tests was equal to 0.33σ (equivalent to five IQ points). Assuming that the instructional materials in the intervention program did not closely resemble Piaget's tests, it is a warranted conclusion that the intervention appreciably raised the level of g.
As in the other studies reviewed here, the specific causal agent has not yet been isolated. Perhaps it never will be, because the intervention effect is most likely the result of a great many small, varied, and unrelated events with beneficial effects that saturate the child's experience throughout an extended period during early development. And perhaps the critical factor is a certain combination of such events. Anything less than very early and intensive intervention, including medical and nutritional advantages, during the preschool years (and also prenatally), is probably inadequate to cause a lasting increase in the child's level of g.