  We describe the principles of counterfactual thinking in providing more
precise definitions of causal effects and some of the implications of this work
for the way in which causal questions in life course research are framed and
evidence evaluated. Terminology is explained and examples of common life course
analyses are discussed that focus on the timing of exposures, the mediation of
their effects, observed and unobserved confounders, and measurement error. The
examples are illustrated by analyses using singleton and twin cohort data.
