Travel behavioral data from five successive waves of the Puget Sound Transportation Panel were examined to determine whether period effects or cohort effects have a significant effect in life-cycle behavior. It was found that period and cohort effects may have a greater influence in household life-cycle models than previously believed. The results bring into question current methods using cross-sectional data analysis for life-cycle models, since predicted changes in activity behavior were not observed in households that made life-cycle transitions. The results are not conclusive, since other variables influencing activity behavior were not accounted for in the analysis.