It begins at conception, when males and females start their developmental journeys down two very different paths. That juncture will affect the way they think, feel, and act for the rest of their lives. The brains of both sexes appear to be “female” until about the eighth week when a male brain is washed by a huge surge of testosterone. It is then transformed radically and even takes on a different color.

This male sex hormone kills some of the communication cells, including a portion of the bundle of nerves called the corpus callosum. It is a rope of fibers that connects the right hemisphere, where emotion is processed, with the left, where language is focused. Although the corpus callosum survives the testosterone bath, the male brain will never be able to “cross talk” as effectively thereafter, which has major implications for future masculine behavior. Testosterone also causes an increase in the volume of neurons and circuits located in the boy’s sex and aggression centers.

Because the female brain is not subjected to a comparable surge of testosterone in the womb and beyond, its communicative and emotional centers remain intact. In fact, these structures will grow larger and become better networked neurologically. A girl’s corpus callosum is up to 25 percent larger than a male’s and becomes an eight-lane superhighway capable of carrying great quantities of emotional information from one side of the brain to the other. (For boys, it is a country road.)

As a result, a girl is likely to be more expressive and emotional than most boys almost from birth. She will probably feel things more deeply and respond to subtle cues in her environment that boys are likely to miss. She will be far better at reading character and motives in others, although she probably won’t be able to explain how she does it. She will also cry more often, even as an adult. Every man knows that, and he is typically rattled by it. She is a girl, after all. That’s what girls do.

(From Dr. James Dobson’s book, Bringing Up Girls.)