<p>Commonly used in employee selection, cognitive ability tests (CATs) show mean score group differences by gender, ethnicity and socioeconomic status (SES). Prior research supports both substantive accounts, such as biological and environmental factors, and methodological and contextual accounts, such as stereotype threat and differential item functioning, related to group differences. This systematic review explores the evidence for the theoretical explanations that purport to explain these differences. Two hundred and twenty-five papers met inclusion criteria which explored potential explanations of group differences. These were grouped into 10 factors: biological, environmental, latent trait / measurement invariance, criterion validity, item bias, behaviour in test, anxiety, attitudes, experience, and stereotype threat. Results showed that whilst biological and environmental factors are related to group differences, so are several factors relevant to test design and administration, most notably stereotype threat, differences in domain experience and exposure and self-confidence. Theoretical and practical implications for the occupational use of CATs as well as limitations and recommendations are discussed.</p>