The presence of extraneous factors in a study that affect the dependent variable and can decrease the internal validity of the study is the ______ variable problem.

An organized family life

A) is linked to economic disadvantage. B) provides a supportive context for warm, involved parent–child interaction. C) guarantees high IQ in offspring. D) can stifle children's creativity and independence.

Identical twins are less alike as adults than they were as children, which demonstrates the influence of

a. heredity. b. nature. c. polygenic factors. d. the environment.