The findings suggest that intelligence can adapt to meet increased environmental demands but when those demands are no longer present, it returns to its previous level.

"While both IQ scores and general intelligence can be raised through targeted environmental interventions, any gains are not permanent and fade over time," said researcher John Protzko from University of California Santa Barbara, US.

The research was published in the journal Intelligence. Protzko reviewed the results of a study, Infant Health and Development Program, involving 985 children, all of whom experienced an intense and cognitively demanding environment during the first three years of their lives. The interventions were employed to ameliorate the negative effects of being born at low birth weight.

At age three, the children were given the Stanford-Binet Intelligence Scales as a baseline measure of their intelligence. At ages five and eight -- at least two years after the interventions had ended -- they were again given intelligence tests.

The results showed that the interventions had raised the children's general intelligence at age three. However, by age five the increases were no longer evident.

He also noted that this difference in intelligence at ages three and five underscored another issue: causality. One theory regarding the development of intelligence suggests that the trait can be correlated between two ages because there is a causal connection: Intelligence at one age causes intelligence at another age.

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