This book provides an excellent overview of research on sex differences, their potential origins, and debates on their implications. It emphasizes how most sex differences manifest in overlapping statistical distributions with small differences rather than Mars vs Venus binary differences. The largest sex differences (e.g., voice pitch) are often uncontroversial, while more contentious topics such as ability often see small differences in mean and variance – knowing an individual’s data will not reliably predict their sex. Unlike gender, biological sex has a clear definition (i.e., eggs vs sperm); human sex differences can be observed over time and across cultures, and often resemble differences seen among other species. Sex differences become controversial because people seek to infer normative implications, where one sex is ‘better’, or existing differences justify particular objectives or policies. However, there is no inherent relationship between normative objectives and research findings. A more nuanced approach to sex differences can be helpful for science and policy debates – for example, a recognition that heart attacks manifest differently in men and women could help improve screening. A review of research on aggression highlights larger age differences than sex differences (toddlers are especially violent), and although acts of violence are more likely to be committed by men, victims are also more likely to be men. The author argues that evolutionary explanations based on the interaction of nature and socialization are compelling to understand sex differences and change over time. These interpretations may sometimes be more suggestive than definite, yet the book is valuable to anyone interested in understanding research on sex differences.