Jonas might perform a correlation study to understand the relationship between head size and intelligence. A correlation study is a statistical method that measures the strength and direction of the relationship between two variables. In this case, the two variables would be head size and intelligence, and Jonas would collect data on both variables for a sample of individuals. The goal of the study would be to determine if there is a statistically significant association between head size and intelligence and, if so, the nature of that relationship.
It is important to note that correlation does not imply causality, meaning that just because two variables are associated with each other does not mean that one causes the other. Thus, it is important to consider other possible explanations for the relationship between head size and intelligence, such as the influence of genetics or other factors, before making any causal claims.