Introduction
Physical development generally refers to a child’s physical characteristics (such as eye or hair color), growth (changes in height, weight, and bodily proportions), motor development (increasing control over movement, as in the progression from sitting up to standing to walking), and brain and nervous system development (an increase in the size and complexity of the brain and an increase in the efficiency of message transfers). Dramatic changes take place in these four areas during the early years of life because of the combined influence of a child’s genetic makeup and the environment.
Genes provide a master plan, or blueprint, for development. Genes are chemical bases that form small sections of thread-like structures called chromosomes. Each chromosome is made of thousands of genes, and every cell in the human body has twenty-three pairs of chromosomes. Encoded in the genes are those traits common to the human race (such as the ability to walk upright) as well as those traits that make each child unique (such as the rate of maturation). Genes direct the form and sequence of development, including setting limits on a child’s potential and predisposing him or her toward either a normal or an abnormal course of development.
Genetic Traits
Parents pass traits along to their offspring in chromosomes. During the process of fertilization, the twenty-three chromosomes in the mother’s ovum unite with the twenty-three chromosomes in the father’s sperm. The resulting baby’s traits depend on which genes from the mother are matched with which genes from the father. In some cases, certain genes are dominant or recessive. For example, the gene for brown eyes is dominant over the gene for blue eyes, which is recessive. If a child inherits the dominant gene for brown eyes from one parent and the recessive gene for blue eyes from the other parent, the child will have brown eyes. A child must receive the recessive gene for blue eyes from both parents to have blue eyes. In other cases, certain traits appear to be a mixture. These polygenic traits are the result of the combined influence of many genes; skin color is one example of a polygenic trait. Traits may also be sex linked, which means that the genes for certain traits (color blindness, for example) are carried in the sex chromosomes. Sex chromosomes constitute the twenty-third pair of chromosomes and function to determine a person’s sex. (Women have two X chromosomes, whereas men have one X and one Y chromosome.)
Environmental Influence
Whereas genes provide a blueprint for development, the environment may modify the extent to which the blueprint is followed by influencing the direction and speed of a child’s physical development and the expression of certain traits. Environmental influences are those that exist outside the person; they generally include a person’s physical and sociocultural surroundings. The physical environment includes such aspects of a child’s immediate environment as the exposure of a fetus to drugs taken by its mother or the exposure of a young child to such environmental hazards as lead or mercury. The sociocultural environment refers to the influence that the people in a child’s life may have on his or her development. For example, the parent-child relationship and the parents’ ethnic backgrounds have been found to influence a child’s physical development.
Genotype and Phenotype Relationship
Although some genotypes (a person’s genetic makeup) are expressed directly in a person’s phenotype (observable traits and behavior), as in the case of eye color or blood type, most phenotypes are the result of an interaction between genes and the environment. Some researchers, in fact, have stated that development is the expression of a person’s genotype in the context of that individual’s environment. For example, children may inherit a genetic tendency toward obesity or toward being athletic. Whether children become obese, however, may depend on the eating behaviors of their family; similarly, whether children develop their full athletic potential may depend on the children’s nutritional status, opportunities available to participate in athletic activities, and the amount of parental support and encouragement received.
Genetic and environmental factors affect many aspects of physical development, ranging from common physical traits and everyday bodily functions to diseases and disorders. Children’s physical features (such as hair and eye color), blood type, amount of body fat, body build, metabolism, weight, health, activity level, sensitivity, blood pressure, timing and rate of maturation, hormonal regulation, sequence of motor development, and hand preference are all examples of genetically based traits. Dominant gene traits include brown eyes, brown hair, curly hair, thick lips, and normal color vision, whereas blue eyes and blond hair are examples of recessive gene traits. Height and weight are two types of polygenic traits; color blindness is an example of a sex-linked trait.
Genetic Cause of Disorders
Defective (or absent) genes can cause genetic disorders and diseases that are acquired through these same inheritance patterns. Achondroplasia (a type of dwarfism that is apparent at birth) and Huntington’s chorea (a genetic disease that affects the central nervous system and causes progressive neural degeneration), for example, are dominant gene disorders. Sickle-cell anemia (a painful blood disorder that often leads to heart or kidney failure), congenital deafness, phenylketonuria (PKU, the lack of an enzyme to complete the metabolism of milk protein), cystic fibrosis (a metabolic error leading to sticky fluid in the lungs that clogs the airways), and galactosemia (defective milk sugar metabolism) are all recessive gene disorders. Many disorders are polygenic; among them are cleft palate and cleft lips, childhood diabetes, spina bifida, hip dislocation, and allergies. Color blindness, hemophilia (the inability of the blood to coagulate), and fragile X syndrome (a leading cause of intellectual disability among newborns) are all sex-linked disorders.
Developmental disorders may also result from faulty chromosome distribution. Down syndrome, for example, results when a child has three instead of the normal two number 21 chromosomes. Children with Down syndrome have distinct physical features and are moderately to severely intellectually disabled. Errors of sex-chromosome distribution can also occur—female children may lack a second X chromosome, or male children may have an extra X or Y chromosome. These sex-chromosome disorders generally result in infertility, some type of intellectual disability, or both.
Environmental Causes of Disorders
Some of the physical, social, and historical influences of the environment begin exerting their effects on a child’s development even before birth. Diseases of the mother during pregnancy, such as syphilis, German measles, herpes, gonorrhea, and acquired immunodeficiency syndrome (AIDS), may have consequences for the fetus ranging from blindness, deafness, and brain damage to death. Also, drugs ingested by the mother during pregnancy (including alcohol and the nicotine in cigarettes) may affect the unborn child in mild or severe ways. Maternal emotional stress, inadequate medical care, incompatible blood types between the mother and the fetus, exposure to high levels of radiation, and malnutrition all have the potential for adversely affecting the developing fetus.
The impact of some of these environmental influences on development (especially drugs, disease, and malnutrition) depends in part on the timing and the dosage involved. Critical periods
are specific times in development when developing organs, structures, or abilities are most vulnerable to environmental influences. During prenatal development, the first three months of pregnancy is a critical period because all the basic structures (the eyes, ears, brain, heart, and limbs) and organ systems of the fetus are being formed. Shortly after birth, there is another critical period for brain development, as the brain is developing rapidly. Serious damage to the fetus or infant can result if it is exposed to harmful environmental influences during this time, because the parts of the body that are developing the most rapidly are the most vulnerable. If exposed to the same influences later, the child would most likely suffer less serious consequences.
After a child is born, health, medical care, nutrition, and disease continue to influence physical development. Exposure to environmental hazards such as lead or toxic mercury levels can also cause deformities, intellectual disability, and poor muscular development and control.
Role of Practice
Normal development not only requires the absence of genetic and environmental difficulties; it also requires practice. In studies of children, normal experience or practice has been found to be necessary for normal motor development. In addition, studies using animals have found that normal, everyday visual experience is necessary for the development of normal vision and that enriched or deprived sensory environments can affect brain development (for example, the weight of the cortex and the number of interconnections between the brain cells). In some instances, experience has also been found to speed up development. Philip Zelazo and his colleagues reported in the early 1970s that practice of the walking reflex early in life led to earlier walking in infants; other studies suggest that such practice can be harmful. Some studies have reported that infants who have experience sitting in someone’s lap are able to sit alone earlier. Although some skills may be influenced by practice, physical and neural maturation limit what can be achieved.
Sociocultural Factors
Sociocultural factors, such as family interaction patterns and ethnicity, affect development in a number of ways. Studies have found, for example, that parental attentiveness and encouragement are important factors influencing whether infants take an interest in and explore their environments. Studies of maternal deprivation have found that mothers who are emotionally neglectful, nonnurturant, cold, and unaffectionate toward their children can predispose their children toward a condition called psychosocial dwarfism. This condition is characterized by growth retardation and abnormally low levels of growth hormone in spite of the fact that these children have received proper food, have been given adequate medical care, and do not have an illness or physical defect. (Researchers believe that emotional deprivation affects normal hormonal functioning in the body, thereby inhibiting growth.) Physical development may also vary across cultures and ethnic backgrounds—the rate of maturation of African infants, for example, is such that they tend to reach motor milestones such as sitting and walking before European or American infants.
Historic Development of Study
The question of the respective contributions of genes and the environment to development reflects a centuries-old debate by philosophers, psychologists, and others concerning human nature: whether inborn differences or the environment contributes more to human development.
Throughout most of the twentieth century, the predominant view in American psychology emphasized the role of the environment as the predominant influence on human development and behavior. At least part of the reason for this may be attributable to a lack of technology available for in-depth investigation and measurement of human biological processes. Advances in science and technology, however, have allowed better research that has greatly increased knowledge about such processes as brain development and its role in normal and abnormal development and behavior; the role of hormones in development, including the regulation of growth, maturation, sex differences, and other developmental processes and behaviors; and the identification of specific genes and their respective functions in normal and abnormal development. Significant advances in knowledge will continue to be made in these areas.
Whereas the genes versus environment debate has historically focused on the relative contribution of each to development, that view has been abandoned in favor of a focus on how genes and the environment interact. This is partly a result of the fact that most developmental psychologists accept that both factors are necessary for development. Robert Plomin’s work in the late 1970s and Sandra Scarr’s work in the early 1980s address this focus by suggesting that a person’s genetic makeup may influence the kinds of environments that he or she experiences. For example, a child who is athletically inclined may spend more time in athletics than in other kinds of activities, receive more attention and encouragement from coaches (compared with those not so gifted), associate frequently with sports-minded people, and (through increased athletic activity) further develop and refine his or her talents. This emphasis on how genes and the environment interact will continue to be a major focus of research in the field of developmental psychology.
Implications for Psychology
The examination of the determinants of physical development is important for a number of reasons. First, it sheds light on the causes (and consequences) of normal and abnormal growth patterns, motor development, and brain and nervous-system development. Second, it may provide insight into some of the causes of normal and abnormal psychological development and behavior because of the relationship between physical development and behavior. Third, it helps explain individual variations in development. Finally, this area of study asserts that human development is not based solely on either the environment or genetics, but rather is caused by the mutual influence of genes and the environment.
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