Chapter 8

Human Development

Human development is the scientific study of changes that occur in people as they age from conception until death. Various research designs are employed to study development, including longitudinal design, where one participant or group is studied over a long period of time; cross-sectional design, where several different participant age-groups are studied at one point in time; and cross-sequential design, which combines these approaches. A cohort effect refers to the impact on development when a group of people share a common time period or life experience.

Nature vs. Nurture

The influence of nature pertains to our inherited characteristics affecting personality, physical growth, intellectual growth, and social interactions, while nurture involves the environmental influence on these areas. Although scientific research acknowledges that behavior is a mix of both, behavioral genetics remains a popular topic, focusing on how much of behavior is attributable to genetic inheritance versus personal experiences. Genetics involves the study of inherited traits, with DNA (deoxyribonucleic acid) being the molecule that carries genetic material. Genes, which are sections of DNA, lead to specific traits through dominant and recessive expressions. A dominant gene controls a trait's expression, while a recessive gene influences a trait only when paired with an identical gene. Genetic transmission is often influenced by multiple gene pairs in a process known as polygenetic inheritance.

Genetic Disorders and Chromosomal Disorders

Genetic disorders can arise from dominant gene disorders, such as Huntington’s disease and Marfan’s syndrome, or recessive disorders, including cystic fibrosis, sickle-cell anemia, Tay-Sachs disorder, and phenylketonuria (PKU). Chromosome disorders occur when individuals have more or fewer than the standard 23 pairs of chromosomes, exemplified by Down syndrome (an extra chromosome on the 21st pair), Klinefelter syndrome (an extra sex chromosome on the 23rd pair, XXY), and Turner syndrome (a missing X chromosome).

Prenatal Development

Prenatal development, spanning conception to birth (about nine months), is affected by numerous environmental factors. Fertilization is the union of the ovum and sperm, resulting in a zygote that undergoes mitosis, eventually forming the baby. Monozygotic (identical) twins arise when one zygote splits into two separate masses, while dizygotic (fraternal) twins form from two separate fertilized eggs. The dangers of high-risk births, including preterm delivery, contrast with the insights gained from studying twins regarding the nature versus nurture debate. In the prenatal stages, three periods of development occur: germinal (first two weeks), embryonic (two to eight weeks), and fetal (eight weeks to birth).

Stages of Life

In the germinal period, the zygote implants in the uterine lining, and the placenta and umbilical cord form. The embryonic period involves the development of major organs, and critical periods can have devastating impacts due to teratogens, substances that can cause birth defects. The fetal period is marked by significant growth and organ development, culminating in full-term birth (38-40 weeks).

Infancy and Childhood Development

The infancy and childhood stages are characterized by rapid growth and development in motor skills, cognitive abilities, and sensory systems. Infants exhibit specific reflexes that aid survival, such as grasping and rooting. Their motor development progresses significantly, paralleled by brain development where unused synaptic connections undergo pruning. Cognitive development follows Jean Piaget's theory of cognitive stages, which include sensorimotor, preoperational, concrete operations, and formal operations. Vygotsky emphasizes social interaction and scaffolding in cognitive development.

Psychosocial Development

Psychosocial development, a concept by Erikson, maps various stages of personality and relationship formation, such as trust versus mistrust in infancy, and identity versus role confusion during adolescence. The development of self-concept and attachment styles, including secure, avoidant, and ambivalent attachments, shapes interpersonal relationships throughout life. The journey through adulthood involves addressing themes of intimacy versus isolation and generativity versus stagnation, alongside challenges associated with aging and health.