Final answer:
Schaie found that while crystallized intelligence remains steady or improves into middle adulthood, fluid intelligence begins to decline in later adulthood. These findings are measured using tests such as the WAIS, which are regularly updated to remain accurate.
Step-by-step explanation:
When Schaie tested adult intelligence using a cross-sequential research design, he found that cognitive abilities remain steady throughout early and middle adulthood. This is in line with the findings that crystallized intelligence, which encompasses knowledge and skills obtained over a lifetime, tends to stay stable or even improve into middle adulthood. However, in later adulthood, there is typically a decline in fluid intelligence, which includes information processing abilities, reasoning, and memory. Tests such as the Wechsler Adult Intelligence Scale (WAIS) have evolved to measure these aspects of intelligence and are continually updated to reflect changes in populations and norms. Research has consistently shown that engaging in mental and physical activities can help delay cognitive decline.