SUMMARY

1. In the first two years of life, infants grow taller, gain weight, and increase in head circumference—all indicative of development. Birthweight doubles by 4 months, triples by 1 year, and quadruples by 2 years, when toddlers weigh about 30 pounds (13½ kilograms).

2. Sleep gradually decreases over the first two years. As with all areas of development, variations are caused by both nature and nurture. Bed-sharing is the norm in many developing nations, and co-sleeping is increasingly common in developed ones.

3. Brain size increases dramatically, from about 25 to 75 percent of adult weight between birth and age two. Complexity increases as well, with proliferating dendrites and synapses. Both growth and pruning aid cognition. Experience is vital for brain development.

4. At birth, the senses already respond to stimuli. Prenatal experience makes hearing the most mature sense. Vision is the least mature sense at birth, but it improves quickly. Infants use all their senses to strengthen their early social interactions.

5. Infants gradually improve their motor skills as they grow and their brains develop. Gross motor skills are soon evident, from rolling over to sitting up (at about 6 months), from standing to walking (at about 1 year), from climbing to running (before age 2).

6. Babies gradually develop the fine motor skills to grab, aim, and manipulate almost anything within reach. Experience, time, and motivation allow infants to advance in all their motor skills.

7. About one billion infant deaths have been prevented in the past half-century because of improved health care. One major innovation is immunization, which has eradicated smallpox and virtually eliminated polio and measles. Public health doctors worry that too many parents avoid immunization, decreasing herd immunity.

8. Breast-feeding is best for infants, partly because breast milk helps them resist disease and promotes growth of every kind. Most babies are breast-fed at birth, but in North America few are exclusively breast-fed for six months, as doctors worldwide recommend.

9. Severe malnutrition stunts growth and can cause death, both directly through marasmus or kwashiorkor and indirectly through vulnerability if a child catches measles, an intestinal disorder, or some other illness.

10. Piaget realized that very young infants are active learners, seeking to understand their complex observations and experiences. Sensorimotor intelligence develops in six stages, beginning with reflexes and ending with mental combinations.

11. Infants gradually develop an understanding of objects. As shown in Piaget’s classic experiment, infants understand object permanence and search actively for hidden objects by about 8 months. Other research finds that Piaget underestimated infant cognition in the timing of object permanence and in many other ways.

12. Another approach to understanding infant cognition is information-processing theory, which looks at each step of the thinking process, from input to output. Modern research benefits from computer analysis and brain scans. The data reveal very active infant minds months before motor skills can demonstrate understanding.

13. Infant memory is fragile but not completely absent. Reminder sessions help trigger memories at 3 months, and by the second year infants remember sequences and object use, learning by observing other people.

14. Language learning may be the most impressive cognitive accomplishment of humans, distinguishing the human species from other animals. Eager attempts to communicate are apparent in the first weeks and months. Infants babble at about 6 to 9 months, understand words and gestures by 10 months, and speak their first words at about 1 year.

15. Vocabulary begins to build very slowly until the infant knows approximately 50 words. Then the naming explosion begins. Toward the end of the second year, toddlers put words together in short sentences, showing that they understand the rudiments of grammar.

16. Various theories explain how infants learn language as quickly as they do. The three main theories emphasize different aspects of early language learning: that infants must be taught, that their social impulses foster language, and that their brains are genetically attuned to language. Each theory seems true for some aspects of language acquisition.