Developmental psychology is the scientific study of the changes that occur as people age from conception to death, using longitudinal, cross-sectional, and cross-sequential studies.
Some of this work focuses on disentangling the relative contributions of genetics and environment in shaping an individual. Genetics is the science of heredity and involves the study of DNA,
genes, and chromosomes. Dominant genes are more likely to influence a trait, whereas recessive genes will only get their instructions carried out if the other chromosome in the pair also
contains a recessive gene.

Fertilization occurs when the sperm penetrates the egg (or ovum). The result is a single cell with 46 chromosomes (23 from the sperm and 23 from the egg). This cell is called a zygote.
The germinal period of pregnancy is the first two weeks after fertilization during which the zygote migrates to the uterus and attaches to the uterine wall. The embryonic period lasts from
about Week 2 to Week 8 after which the embryo is about one inch long with primitive eyes, nose, lips, teeth, arms and legs. The fetal period lasts from the eighth week after conception to
the end of the pregnancy.

The brain triples in weight during the first two years of life, caused by the expansion of existing cells rather than the growth of new ones. Jean Piaget believed that the primary factor in the
development of a child’s cognitive abilities was the child’s interactions with objects in the environment. He proposed four stages of cognitive development from infancy to adolescence:
sensorimotor, preoperational stage, concrete operational, and formal operational. Vygotsky felt the primary factor in development was the social environment. He proposed concepts such
as scaffolding and the zone of proximal development.

Temperament refers to behavioral and emotional characteristics observed in infancy. Attachment is the emotional bond between an infant and caregiver. Mary Ainsworth and others studied
attachment using the Strange Situation and observed four attachment styles, secure, avoidant, ambivalent, and disorganized/disoriented. Erik Erikson proposed an eight-stage theory of
development that occurred over the entire life span. Each stage involved an emotional crisis in the individual’s social interactions. The first four stages occurred during infancy and childhood
and consisted of the crises of trust versus mistrust, autonomy versus shame and doubt, initiation versus guilt, and industry versus inferiority.

Adolescence is the period of time from around age 14 to the early 20s and is most clearly identified by the physical changes that occur in puberty. Mentally, many adolescents are moving
into Piaget’s formal operational stage of development, which includes the ability to think in the abstract and to consider hypothetical situations. At the same time, adolescents still demonstrate
a considerable amount of egocentric thinking as can be seen in the thought processes of the personal fable and the imaginary audience.

Lawrence Kohlberg proposed a theory about the development of moral thinking and divided the development into three levels, pre-conventional, during which the individual conforms
to social norms; conventional, during which time the consequences determine morality; and post-conventional, during which a person’s individual moral principles determine right and wrong.
The social crisis proposed by Erikson for the period of adolescence is that of identity versus role confusion.

Adulthood can be roughly identified as the time period from the early 20s until death. Cognitive abilities do not decline overall but the speed of processing does appear to slow down.
Parenting is a significant part of many people’s adulthood. Diana Baumrind proposed three basic parenting styles, authoritarian, permissive, and authoritative. There are a number of theories
regarding why our bodies physically age: The cellular clock theory, the wear-and-tear theory, and the free radical theory are chief among them.

Based on her work with dying patients, Elizabeth Kübler-Ross proposed a sequence of five emotions (denial, anger, bargaining, depression, and acceptance) that occur during the process
of dying.