Final answer:
Improved cognitive abilities, understanding of others' intentions, and increased parental involvement and supervision lead to declines in aggression across the preschool years.
Step-by-step explanation:
Factors that lead to declines in aggression across the preschool years include improved cognitive abilities and understanding of others' intentions. This is because as children develop better cognitive skills and social understanding, they are able to communicate their needs and resolve conflicts without resorting to aggression. Additionally, increased parental involvement and supervision can create a nurturing environment that teaches children alternative ways of expressing their emotions and resolving conflicts without resorting to aggression.