Foster child method 2-1

 Placing children at early age to avoid influence of original home

Child selection must be random.

Sample size should be large 

Children should be from one population to eliminate ethnic variations.

Good homes 112 IQ

Average 105 and poor 96

Demonstrates effect of environment on intelligence.

The most cited adoption projects that sought to estimate the heritability of IQ were those of Texas,[6] Colorado[7] and Minnesota[8] that were started in the 1970s. These studies showed that while adoptive parents IQ does seem to have a correlation with adoptees IQ in early life, when the adoptees reach adolescence the correlation has faded and disappeared

Environment variations begin even before birth and difficult to control during the experiment. Epigenetic effects of smoking and alcohol.

Ethical concerns of playing with children's lives.