Anna Machin, an evolutionary anthropologist at Oxford who has studied fatherhood for over a decade, reveals the biological, psychological, and social transformations men undergo when they become fathers. Machin draws on her research with expectant and new fathers to show that fatherhood is not merely a social role but a developmental experience that changes men's brains and hormones. Testosterone decreases, promoting nurturing behavior; oxytocin increases during father-child interaction; and brain regions associated with empathy and attachment strengthen. The book examines what fathers uniquely contribute to children's development: rough-and-tumble play that teaches risk assessment and emotional regulation, challenging interactions that promote resilience, and models of masculine behavior that shape children's expectations. Machin addresses how fatherhood varies across cultures and how modern expectations differ from evolutionary precedent without invalidating either. She explores the particular challenges facing contemporary fathers: workplace structures that assume primary caregivers are female, cultural narratives that treat fathers as secondary parents, and the absence of institutional support that mothers receive. The book combines scientific findings with practical guidance, speaking to fathers themselves as well as mothers, employers, and policymakers. Machin writes accessibly about complex research, making the case that investing in fatherhood benefits children, families, and society.