In this work, Jerome Kagan demonstrates that innovative research methods in the behavioural sciences and neurobiology, together with a renewed commitment to rigorous empiricism, are transforming our understanding of human behaviour. Kagan argues that behavioural scientists have reached less-than-satisfactory answers to the fundamental questions about temperament, cognition and the self because they have failed to appreciate the biases inherent in their frame of reference and the limitations of their investigative procedures.