Chapter Overview

Prenatal Development

• Developmental processes begin at the moment of human conception. The period from conception to birth is called prenatal development. This period of development is mainly directed by genetic influences and follows a universal pattern.

• Genes and chromosomes are the basic building blocks of human life. Each cell contains 23 pairs of chromosomes, which are composed of DNA structures containing thousands of genes. Most human traits are caused by dominant genes, but some characteristics are also caused by recessive genes. Many human characteristics are polygenetic, meaning they result from the combination of more than one gene.

• The twenty-third chromosome determines the sex of the child. Females have two X chromosomes, whereas males have one X and one Y chromosome. Traits carried on the sex chromosome are called sex-linked characteristics. Males have a higher likelihood of baldness, blindness, and hemophilia, because these traits are carried on the female X chromosome, and there is no corresponding dominant gene on the Y chromosome.

• Every person is a carrier of at least 20 genes that can produce genetic disorders and diseases. Most genetic disorders are caused by recessive genes. Common genetic disorders include hemophilia, sickle-cell anemia, PKU, muscular dystrophy, congenital diabetes, and cystic fibrosis. Some genetic disorders are caused by abnormal chromosomes. Down syndrome and fragile-X syndrome are two conditions caused by an extra or a damaged chromosome. Both conditions affect the mental and physical development of the child.

• The genes a person inherits from both parents determine the person's genotype from a particular trait. The expression of these traits is called a phenotype. The person's genotype and phenotype can differ when the genotype includes a dominant and recessive gene. The environment can also affect the expression of a particular genotype. If the environment does not facilitate the expression of a particular trait (e.g., high intelligence) then the genotype and phenotype will differ. The concept of reaction range defines the degree to which the environment can affect the expression of genes.

• Prenatal development is divided into three major stages. During the germinal period (first two weeks after conception), the fertilized egg travels down the fallopian tube and attaches itself to the uterine wall. In the embryonic period (weeks 2 to 8), all the basic structures of the child-to-be are formed. This prenatal stage is particularly vulnerable to environmental agents because of the rapid development of new organs and systems. During the fetal period (8 weeks to birth), the fetus increases in size and weight. Most babies are born between 38 and 42 weeks.

• Although prenatal development is mainly guided by genetic influences, exposure to dangerous environmental agents or conditions can disrupt this process. Alcohol, nicotine, drugs, radiation, and environmental pollutants can have a negative effect on prenatal development and must be avoided during pregnancy. The fetus is most vulnerable to environmental influences in the early stages of pregnancy when body structures and major organs are developing.

• A premature birth or low birth weight are risk factors in a child's development. Premature births and low birth weights are caused by poor nutrition, poor prenatal care, and environmental agents such as nicotine, alcohol, and drugs. In elementary school, children with a low birth weight are more likely than children of normal birth weight to have mild learning disabilities, attention problems, poor concentration, language delays, and asthma.

Brain Development

• The human brain is not fully mature until early adulthood. Brain development after birth involves increases in the number and length of neural fibers that connect nerve cells, and deposits of a fatty substance (myelin) on neural fibers, which enable impulses to travel faster through the brain. Areas of the brain are programmed to receive certain environmental stimulation. Neurons that receive this stimulation form connections, whereas those that do not receive the appropriate stimulation fade and die off.

• The human brain is most open to change in early development. It loses some of its plasticity once the two hemispheres of the brain begin to specialize. This specialization is known as lateralization. The left hemisphere of the brain controls language processes, whereas the right hemisphere processes visual and spatial information. Researchers have not found a link between hemispheric specialization and learning disabilities or cognitive styles.

• The infant brain quickly develops the perceptual abilities it needs to process and interpret information from the five senses. Babies achieve adult levels of vision by 12 months and prefer visual stimulation that is complex and novel, which stimulates their cognitive development. Auditory perception is quite developed at birth, and newborns are quite adept at discriminating different human sounds. Babies also show preferences for certain smells and tastes, and they respond to touch and pain.

Motor Development

• Motor development involves children's increasing ability to move and to control their body movements. A newborn's movements are mainly due to innate reflexes, such as sucking, choking, and grasping. Gross motor skills that involve the movement of head, body, legs, and arms develop first. Most infants begin to walk between 8 and 10 months. By the time they enter school, children can ride bikes, skip, catch a ball with two hands, and play kickball. Fine motor skills develop in the preschool years. By age 5 or 6, children can copy simple geometric shapes, button their shirts, tie their shoes, and print numbers and letters. Manipulative skills are not fully developed until around the age of 10 or 12. Gender differences in gross and fine motor skills are evident in childhood due to differences in socialization experiences.

• Play is the medium by which children develop their motor skills. Preschool and young elementary schoolchildren need daily activities that exercise their large muscles and help develop their fine motor skills. Older elementary children are able to sit for longer periods of time, but they also need frequent periods of activity. As motor skills are developing, children's play activities need careful supervision. Accidents are the number one cause of childhood deaths.

Physical Growth

• Infancy is characterized by a rapid period of growth. In the elementary school years, physical growth slows down, although children continue to gradually add weight and height to their bodies. In general, African-American children tend to be taller than children of European, Asian, or Hispanic decent. There are few gender differences in physical size until adolescence.

• Children's physical size and attractiveness can influence how they are perceived by others. Children who are small for their age tend to be judged as less mature and competent than children of average height. Children and adults attribute more positive attributes and characteristics to physically attractive than unattractive children. Children who are physically attractive also have more positive peer relations.

• Puberty begins between the ages of 10 and 12 for girls and 12 and 14 years for boys. Within each sex, there is considerable individual variability in the timing of puberty, but the sequence of change is universal. The changes that occur in puberty primarily prepare the body for sexual reproduction, but the development of secondary characteristics is only one of the many changes that are taking place. There are changes in the skeleton, muscles, and vital organs.

• Puberty can pose adjustment problems for adolescents. It often leads to changes in the adolescent's self-image, self-confidence, social relations, moods, and so on. Parts of the body mature at different rates, which can make adolescents feel clumsy and awkward. Adolescents are moody, but there is little evidence to suggest that their mood swings are caused by "raging hormones." The moods of adolescents fluctuate as they change social settings. There is more conflict between children and adults at puberty, but the conflict dissipates as the different parties renegotiate roles, rules, and expectations. Adolescents are more likely to rebel when parents are too controlling and unable to adjust their expectations.

• Early and late maturity have different effects for girls than for boys. Early maturity in girls can lead to low self-esteem, depression, anxiety, and eating disorders. If early-maturing girls seek out the company of older peers, it can also lead to risk behaviors such as drinking, early sexual activity, and delinquency. Early-maturing boys seem to have the advantage in terms of their self-esteem, popularity, and leadership skills. However, if they associate with older peers, they may also be at risk for engaging in deviant or antisocial behavior. In general, recent studies suggest that late-maturing boys and girls experience fewer adjustment problems in the long run than do early maturers.

Special Health Concerns

• Young people, especially adolescents, face more health risks than ever before. In the last 20 years, the health of young people has declined by 50 percent, due to poverty. Most young people are able to make a healthy transition to adulthood, but the numbers of young people who are at risk for serious health problems are increasing.

• Obesity is the most common eating disorder. It is estimated that 11 percent of youths are overweight. A number of factors contribute to childhood obesity, including heredity, and unhealthy diet, lack of physical activity, and television viewing. Obese children experience peer rejection, low self-esteem, and other behavioral problems. They also have a high likelihood of becoming obese adolescents and adults. Schools can help obese children by encouraging healthy eating habits, eliminating junk food, increasing physical activity, and reducing stereotypes of overweight people.

• Anorexia and bulimia nervosa affects 3 percent of adolescents and young women. The girls who are most at risk for these eating disorders are those who have accepted society's definition of feminine attractiveness. Both anorexia and bulimia are associated with low self-esteem and depression and can be life-threatening if left untreated. Medical treatment is generally accompanied by family therapy focused on changing parent-child interactions and expectations.

• The four most frequently abused substances among young people are tobacco, alcohol, marijuana, and inhalants. Young people who abuse alcohol and drugs experience problems in school as well as psychological and health problems. Childhood and adolescence is an important time for the prevention of substance abuse. The most successful prevention programs educate young people about the risk of drug use but also promote interpersonal skills and self-esteem.

• Sex is very much a part of the adolescent experience. Adolescents begin to experiment with their sexuality earlier than ever before. By age 16, one-third or more of young people have already had sexual intercourse. By age 20, 1 out of 4 young people will have contracted a sexually transmitted disease, and 1 out of 3 teenage girls will have become pregnant. The leading causes of these problems are lack of sex education and lack of access to contraceptives. Sex education through the schools has had little effect. Most schools do not teach sex education until it is too late, and most programs do not provide information or access to contraceptives. Furthermore, few programs teach young people the decision-making or interpersonal skills they need to assert themselves in sexual matters.

• A small number of children and adolescents suffer from chronic depression. The symptoms are inability to concentrate, changes in weight, sleep problems, lack of motivation and energy, feelings of hopelessness, and thoughts of death. Depression is more prevalent in adolescents than in children, due to the adolescent's ability to be self-reflective and self-critical. Depression is also more common in girls than in boys during adolescence. In extreme cases, depression can increase a young person's risk of suicide, which is now the second leading cause of death among young people. Researchers believe that schools must play a larger role in preventing these problems. Prevention efforts should focus on improving social skills, teaching stress management strategies, and promoting basic mental health.