chapter summary:

Nature and Nurture

• The complex interplay of nature and nurture was the constant theme of this chapter. In the drama of development, genotype, phenotype, and environment all play starring roles, and the plot moves forward as they interact in many obvious and many not-so-obvious ways.
• The starting point for development is the genotype—the genes inherited at conception from one’s parents. Only some of those genes are expressed in the phenotype, one’s observable characteristics. Whether some genes are expressed at all is a function of dominance patterns. Most traits studied by developmental scientists are influenced by multiple genes. The switching on and off of genes over time underlies many aspects of development. This process is affected by experience via methylation.
• The eventual outcome of a given genotype is always contingent on the environment in which it develops. Parents and their behavior toward their children are a salient part of the children’s environment. Parents’ behavior toward their children is influenced by their own genotypes. Similarly, the child’s development is influenced by the aspects of the environment he or she seeks out and the different responses the child’s characteristics and behavior evoke from other people.
• The field of behavior genetics is concerned with the joint influence of genetic and environmental factors on behavior. Through the use of a variety of family-study designs, behavior geneticists have discovered a wide range of behavior patterns that “run in families.” Many behavior geneticists use heritability estimates to statistically evaluate the relative contributions of heredity and environment to behavior.

Brain Development

• A burgeoning area of developmental research focuses on the development of the brain—the most complex structure in the known universe. Neurons are the basic units of the brain’s informational system. These cells transmit information via electrical signals. Impulses are transmitted from one neuron to another at synapses.
• The most human part of the human brain is the cortex, because it is involved in a wide variety of higher mental functions. Different areas of the cortex are specialized for general behavioral categories. The cortex is divided into two cerebral hemispheres, each of which is specialized for certain modes of processing, a phenomenon known as cerebral lateralization.
• Brain development involves several processes, beginning with neurogenesis and differentiation of neurons. In synaptogenesis, an enormous profusion of connections among neurons is generated, starting prenatally and continuing for the first few years after birth. Through synaptic pruning, excess connections among neurons are eliminated.
• Experience plays a crucial role in the strengthening or elimination of synapses and hence in the normal wiring of the brain. The fine-tuning of the brain involves experience-expectant processes, in which existing synapses are preserved as a function of stimulation that virtually every human encounters, and experience-dependent processes, in which new connections are formed as a function of learning.
• Plasticity refers to the fact that nurture is the partner of nature in the normal development of the brain. This fact makes it possible in certain circumstances for the brain to rewire itself in response to damage. It also makes the developing brain vulnerable to the absence of stimulation at sensitive periods in development.
• The ability of the brain to recover from injury depends on the age of the child. Very early damage, during the time when neurogenesis and synaptogenesis are occurring, can have especially devastating effects. Damage during the preschool years, when synapse elimination is occurring, is less likely to have permanent harmful effects.

The Body: Physical Growth and Development

• Humans undergo a particularly prolonged period of physical growth, during which growth is uneven, proceeding more rapidly early in life and in adolescence. Secular trends have been observed in increases in average height and weight.
• Food preferences begin with innate responses by newborns to basic tastes, but additional preferences develop as a result of experience. Problems with the regulation of eating are evident in the United States, where an epidemic of obesity is clearly related to both environmental and genetic factors.
• In much of the rest of the world, the dominant problem is getting enough food, and nearly half of all the children in the world suffer from undernutrition. Inadequate nutrition is closely associated with poverty, and it leads to a variety of behavioral and physical problems in virtually every aspect of the child’s life. Prevention of undernutrition is needed to allow millions of children to develop normal brains and bodies.