A researcher decides to investigate whether there are any significant changes in personality from early adulthood through late adulthood. The researcher begins with a sample of 21-year-olds and plans to follow this group until they are 80 years old. The type of research design is:

Longitudinal research design

Explanation:

Longitudinal research design is an observational design where data is collected for the same subjects over a long period of time which can range from months to years.

Therefore, in our case the researcher begins with a sample of 21 year olds and is planning to follow this group until they are 80 years in order to investigate whether there are any significant changes in personality from early adulthood through late adulthood.

This makes his research design to be Longitudinal research.