When an intervention to raise intelligence ends, effects fade away; recursive processes between IQ & the environment are not as strong as once thought; intelligence adapts to environmental demands, both upwards & downwards

The environment in raising early intelligence: A meta-analysis of the fadeout effect. John Protzko. Intelligence, Volume 53, November–December 2015, Pages 202-210. https://doi.org/10.1016/j.intell.2015.10.006

Highlights
•    We meta-analyze 39 RCTs that raised children's IQ and followed them after the study ended.
•    We confirm that after an intervention that raises intelligence ends, the effects fade away.
•    The fadeout effect occurs because those in the experimental group lose their IQ advantage.
•    This suggests that recursive processes between IQ and the environment are not as strong as once thought.
•    We propose that intelligence adapts to environmental demands, both upwards and downwards.

Abstract: Many theories about the role of the environment in raising IQ have been put forward. There has not been an equal effort, however, in experimentally testing these theories. In this paper, we test whether the role of the environment in raising IQ is bidirectional/reciprocal. We meta-analyze the evidence for the fadeout effect of IQ, determining whether interventions that raise IQ have sustained effects after they end. We analyze 7584 participants across 39 randomized controlled trials, using a mixed-effects analysis with growth curve modeling. We confirm that after an intervention raises intelligence the effects fade away. We further show this is because children in the experimental group lose their IQ advantage and not because those in the control groups catch up. These findings are inconsistent with a bidirectional/reciprocal model of interaction. We discuss explanations for the fadeout effect and posit a unidirectional–reactive model for the role of the environment in the development of intelligence.