Complex sentence: She states that many studies of psychological gender differences are controversial and subject to error, observing that many results from smaller scale studies are not repeated in larger studies and that self-report questionnaires are vulnerable to bias.
Split into simpler sentences: She states that many studies of psychological gender differences are controversial and subject to error. Observing that many results from smaller scale studies are not repeated in larger studies and that self-report questionnaires are vulnerable to bias.