Richard Lynn and Tatu Vanhanen constructed IQ estimates for many countries using literature reviews, international student assessment studies and other methodologies to create estimates, which have been widely criticized on theoretical and methodological grounds. Subsequent research by psychologists such as Earl B. Hunt, Jelte Wicherts and Heiner Rindermann has focused on identifying potential national differences in cognitive ability and causal factors, and determining the nature of the relationship of IQ to variables such as GDP, life expectancy, and governance.