Final answer:
A reasonable explanation for why 40-year-olds and 60-year-olds show differences in intelligence test scores could be cognitive decline.
Step-by-step explanation:
A reasonable explanation for why 40-year-olds and 60-year-olds show differences in intelligence test scores could be cognitive decline. While our cognitive abilities remain steady throughout early and middle adulthood, in late adulthood, we begin to experience a decline in fluid intelligence, which includes information processing abilities, reasoning, and memory.
This decline is a natural part of the aging process, and it can affect test scores. However, it's important to note that not all individuals experience the same level of cognitive decline, and factors like mental and physical activity can play a role in delaying the onset of decline.