Final answer:
Cohort effects are responsible for differences between cross-sectional and longitudinal findings in Schaie's research on verbal ability.
Step-by-step explanation:
In Schaie's research on verbal ability, the factor responsible for differences between cross-sectional findings and longitudinal findings is A. Cohort effects. Cross-sectional research compares different age groups at the same time, while longitudinal research follows the same individuals over time. Cohort effects refer to the influence of the social and cultural experiences of different generations, which may make them different from one another and introduce differences in the research findings.