What does longitudinal research on IQ throughout adulthood usually find?

Have scientists found the source of g?

1. What does cross-sectional research on IQ throughout adulthood usually find?

2. What does longitudinal research on IQ throughout adulthood usually find?

3. How do historical changes affect the results of longitudinal research?

4. How does cross-sequential research control for cohort effects?

5. Why does IQ vary as much as it does?