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face, voice, and touch. They show a preference for the voice and odor of their mothers. Their vision is best for objects normally about the distance from the infant’s eyes to the caretaker’s face. They can track objects with their eyes when they are only a few days old. Infants can distinguish among different colors, and they prefer certain complex patterns, such as the human face. Newborns also prefer sweet and salty tastes. Their sense of hearing is well developed at birth and typically the dominant sense during the baby’s first months. As structures in the eye and brain develop during infancy, visual acuity (clarity of vision) and depth perception improve, so that sight normally becomes a more dominant sense sometime during the second half of the baby’s first year.

How do psychologists know this information if babies can’t talk? Psychologists depend on gazes, sucking, and head turning measured by sophisticated computerized equipment in response to changes in stimuli to reveal abilities of infants. For example, when infants are shown a stimulus for the first time, they gaze at it for a length of time. With repeated presentations of that stimulus, they look away sooner. If a new stimulus is presented, and the infant can remember and discriminate between the two stimuli, the infant will look at the new stimulus longer than if it perceives no difference between the first and second stimulus. These are called habituation studies. Habituation is decreasing responsiveness with repeated presentation of the same stimulus.

The First Two Years

An infant’s physical development during the first 2 years is amazing. Brain development proceeds rapidly from the prenatal period, during which about 20 billion brain cells are produced, through the baby’s first 2 years, during which dendrites proliferate in neural networks, especially in the cerebellum, then in occipital and temporal lobes as cognitive abilities grow. Body proportions change as the torso and limbs grow more quickly, so that the head is less out of proportion to body size. Physical development of the musculoskeletal system from head to tail, and from the center of the body outward, accompanies nervous system maturation to enable the baby to lift its head, roll over, sit, creep, stand, and walk, normally in that order. Maturation, motor and perceptual skills, motivation, and environmental support all contribute to development of new behaviors. During childhood, proliferation of dendrites proceeds at a rapid rate, especially in the frontal cortex.

Adolescence

The next growth spurt comes in adolescence, following a dramatic increase in production of sex hormones. The defining feature of puberty is sexual maturation, marked by the onset of the ability to reproduce. Primary sex characteristics, reproductive organs (ovaries and testes) start producing mature sex cells, and external genitals (vulva and penis) grow. So do secondary sex characteristics—nonreproductive features associated with sexual maturity—such as widening of hips and breast development in females, growth of facial hair, muscular growth, development of the “Adam’s apple,” and deepening of the voice in males, and growth of pubic hair and underarm hair in both. Girls begin their growth spurt about 2 years before their first menstrual period (menarche), typically at age 12½. Early maturation of females can put them at a social disadvantage, whereas early maturation of males can put them at a social advantage. Boys start their growth spurt about 2 years later than girls, but about 2 years before ejaculation of semen with viable sperm. During adolescence, changes in the brain include selective pruning of unused dendrites with further development of the emotional limbic system, followed by frontal lobe maturation.

Aging

By our mid-20s, our physical capabilities peak, followed by first almost imperceptible, then accelerating, decline. According to evolutionary psychologists, peaking at a time when both males and females can provide for their children maximizes chances of survival for our species. Decreased vigor,