Eleanor Emmons Maccoby, a distinguished psychologist and a pioneer in the field of gender studies who was the first woman to head the Stanford University psychology department, died on Dec. 11 in Palo Alto, Calif. She was 101.

Her death, at a retirement community, was confirmed by her son, Mark, who said the cause was pneumonia.

Dr. Maccoby, whom the American Psychological Association listed among the 100 most eminent psychologists of the 20th century, conducted pathbreaking research in child development and gender studies. She explored a wide range of topics, including interactions between parent and child and the effect of divorce on children.

But the overarching themes of her long career were the differences between the sexes and how they develop. These were the subjects of two of her most significant books: “The Psychology of Sex Differences” (1974) and “The Two Sexes: Growing Up Apart, Coming Together” (1998).