Secondary sex characteristics are physical characteristics that differentiate between adult males and females but are not directly involved in reproduction. Examples include differences in the distribution of body hair, fat, and muscle; size and appearance of external genitalia; breast size; skin texture; size and shape of face and extremities; and voice quality and pitch. Some researchers have argued that sex differences in brain anatomy should also be considered secondary sex characteristics. Secondary sex characteristics become noticeable during puberty, when hormonal changes trigger a wide range of physical changes in both boys and girls. In contrast, primary sex characteristics (the presence of a penis and testes in males, a vagina and ovaries in females) are directly involved in reproduction and are present at birth. In later ...