A pamphlet created to encourage women to stop smoking states that ""Children of mothers who smoked during pregnancy scored nine points lower on intelligence tests at ages three and four than children of nonsmokers."" Suggest some lurking variables that may help explain the association between smoking during pregnancy and children’s later test scores.

Stress Level in mother's lives and surrounding people's (family, peer) addictive habits : could be lurking variables between smoking during pregnancy and children’s later test scores.

Explanation:

Regression is a statistical tool that establishes relationship between independent (causal) variable(s) & dependent (effected) variables.

Lurking Variable is a variable which seems to effect both independent variables, and also defines the relationship between them.

Given Case : Smoking during pregnancy is the independent variable, affecting the dependent variable i.e test scores of children.

Lurking Variables (affecting both mothers' smoking, children's score & their relationship) in this case could be :

Stress Level in mother's lives : As it effects both the mother's additive habits (smoking); & also the the mental growth of baby.
Surrounding people's addictive habits : As it effects the mother's addictive habits (smoking) ; & also the ambience around mother, mental growth of baby.