3.6 Summary

Growth in Infancy

1. In the first two years of life, infants grow longer, gain weight, and increase in head circumference—all indicative of development. Birth weight doubles by 4 months, triples by 1 year, and quadruples by 2 years, when toddlers weigh about 12 to 14 kilograms. Similarly, length increases by more than 30 centimetres in the second year of life, making 2-year-olds half their adult height.

2. Brain size increases even more dramatically, from about 25 to 75 percent of adult weight in the first two years. Complexity increases as well, with cell growth, development of dendrites, and formation of synapses. Both growth and pruning aid cognition. Experience is vital for brain development.

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

Perceiving and Moving

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 begin to grow and as brain maturation increases. 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.

Surviving in Good Health

7. About 2 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. More medical professionals are needed to prevent, diagnose, and treat the diseases that still cause many infant deaths in developing nations.

8. Breastfeeding is best for infants, partly because breast milk helps them resist disease and promotes growth of every kind. Most babies are breastfed at birth, but in North America only one-third are exclusively breastfed for three 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.

Infant Cognition

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 begin to search for hidden objects at 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. Each week, infants understand more about numbers, objects, patterns of speech, and so on.

13. Infant memory is fragile but not completely absent. Reminder sessions help trigger memories, and young brains learn motor sequences long before they can remember with words.

Language

14. Language learning may be the most impressive cognitive accomplishment of infants, 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 child knows approximately 50 words. Then the naming explosion begins. Toward the end of the second year, toddlers put words together, 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 learning, and that their brains are genetically attuned to language. Each theory seems true for some aspects of language acquisition.