Developmental psychology
Developmental psychology, also known as human development, is the scientific study of progressive psychological changes that occur in human beings as they age. Originally concerned with infants and children, the field has expanded to include adolescence and more recently, aging, and the entire life span. This field examines change across a broad range of topics including motor skills and other psycho-physiological processes, problem solving abilities, conceptual understanding, language acquisition, moral understanding, and identity formation.
Developmental psychologists investigate key questions, such as whether children are qualitatively different from adults or simply lack the experience that adults draw upon. Two   important issues concern the nature of development. One concerns whether development occurs through the gradual accumulation of knowledge or through shifts from one stage of thinking to another. The other concerns whether children are born with innate knowledge or figure things out through experience. A third significant area of research examines social contexts that affect development.
Developmental psychology informs several applied fields, including: educational psychology, child psychopathology and forensic developmental psychology.  Developmental psychology complements several other basic research fields in psychology including social psychology, cognitive psychology, cognitive development, and comparative psychology.

# Theory
Many theoretical perspectives attempt to explain development; among the most prominent are: Jean Piaget's Stage Theory, Lev Vygotsky's Social Contextualism (and its heir, the Ecological Systems Theory of Urie Bronfenbrenner), and the information processing framework employed by cognitive psychology.
To a lesser extent, historical theories continue to provide a basis for additional research. Among them are Erik Erikson's eight stages of psychosocial development and John B. Watson's and B. F. Skinner's behaviorism. Many other theories are prominent for their contributions to particular aspects of development.  For example, attachment theory describes kinds of interpersonal relationships and Lawrence Kohlberg describes stages in moral reasoning.  Human development is also an area of study in education. One of the pioneers in defining the stages of human development was Robert J. Havighurst. His major contribution was defining the developmental tasks for six basic age groups.

## Role of experience
A significant question in developmental psychology is the relation between innateness and environmental influence in regard to any particular aspect of development. This is often referred to as "nature versus nurture" or nativism versus empiricism. A nativist account of development would argue that the processes in question are innate, that is, they are specified by the organism's genes. An empiricist perspective would argue that those processes are acquired in interaction with the environment. Today developmental psychologists rarely take such extreme positions with regard to most aspects of development; rather they investigate, among many other things, the relationship between innate and environmental influences. One of the ways in which this relationship has been explored in recent years is through the emerging field of evolutionary developmental psychology.
One area where this innateness debate has been prominently portrayed is in research on language acquisition. A major question in this area is whether or not certain properties of human language are specified genetically or can be acquired through learning. The nativist position argues that the input from language is too impoverished for infants and children to acquire the structure of language. Linguist Noam Chomsky asserts that, evidenced by the lack of sufficient information in the language input, there is a universal grammar that applies to all human languages and is pre-specified. This has led to the idea that there is a special cognitive module suited for learning language, often called the language acquisition device.
The empiricist position on the issue of language acquisition suggests that the language input does provide the necessary information required for learning the structure of language and that infants acquire language through a process of statistical learning. From this perspective, language can be acquired via general learning methods that also apply to other aspects of development, such as perceptual learning. There is a great deal of evidence for components of both the nativist and empiricist position, and this is a hotly debated research topic in developmental psychology.
On the other hand, Chomsky's critique of a specific empiricist position on this issue, radical behaviorist Burrhus Frederic Skinner's Verbal Behavior written in 1957, is widely considered among developmental psychologists to have sparked the decline in influence of behaviorism and signaled the beginning of the cognitive revolution in psychology.

## Mechanisms of development
Developmental psychology is concerned not only with describing the characteristics of psychological change over time, but also seeks to explain the principles and internal workings underlying these changes. Understanding these factors is aided by the use of models. Developmental models are often computational, but they do not necessarily need to be. A model must simply account for the means by which a process takes place. This is sometimes done in reference to changes in the brain that may correspond to changes in behavior over the course of the development. Computational accounts of development often use either symbolic, connectionist (neural network), or dynamical systems models to explain the mechanisms of development.

# History of developmental psychology
The modern form of developmental psychology has its roots in the rich psychological tradition represented by Aristotle, Tabari, Rhazes, Alhazen, and Descartes. William Shakespeare had his melancholy character Jacques (in As You Like It) articulate the seven ages of man: these included three stages of childhood and four of adulthood. In the mid-eighteenth century Jean Jacques Rousseau described three stages of childhood: infans (infancy), puer (childhood) and adolescence in Emile: Or, On Education. Rousseau's ideas were taken up strongly by educators at the time.
In the late nineteenth century, psychologists familiar with the evolutionary theory of Darwin began seeking an evolutionary description of psychological development; prominent here was G. Stanley Hall, who attempted to correlate ages of childhood with previous ages of mankind.
A more scientific approach was initiated by James Mark Baldwin, who wrote essays on topics that included Imitation: A Chapter in the Natural History of Consciousness and Mental Development in the Child and the Race: Methods and Processes. In 1905, Sigmund Freud articulated five psychosexual stages. Later, Rudolf Steiner articulated stages of psychological development throughout human life. The first three of these stages, which correspond closely with Piaget's later-described stages of childhood, were first presented in Steiner's 1911 essay The Education of the Child. By the early to mid-twentieth century, the work of Vygotsky and Piaget, mentioned above, had established a strong empirical tradition in the field.

## The role of mothers
Traditionally mothers (and women generally) were emphasized to the exclusion of other caregivers. This has begun to change, with the emphasis now placed on a primary caregiver (regardless of gender or biological relation), as well as all persons directly or indirectly influencing the child (the family system). However, studies are showing that the role of the mother/father are more significant than first thought as we moved into the concept of primary caregiver.

## The role of fathers
Because the traditional role of the father was more the breadwinner, and less the direct caregiver of an infant, he has been traditionally viewed as impacting an infant indirectly through interactions with the mother of the child.
However, in a study published in Child Development May/June 2003, Volume 74, Number 3, Pages 801-821, Bruce J. Ellis, et al, found that presence of the natural father was the most significant factor in reducing rates of early sexual activity and rates of teenage pregnancy in girls.  The study found that early father absent girls had the highest rates, with late father absent girls second, and with father present girls having the lowest rates of early sexual activity and preteen pregnancy.  Covariate factors used included early conduct problems, maternal age at first childbirth, race, maternal education, father's occupational status, family living standards, family life stress, early mother-child interaction, measures of psychosocial adjustment and educational achievement, school qualifications, mood disorder, anxiety disorder, suicide attempts, violent offending, and conduct disorder.  With the single exception of GPA in the USA data set, all covariant factors were found to be dichotomous with respect to the father absence factor and the behavior studied through chi-squared analysis.  Studies were conducted in the USA and New Zealand.
Studies  have shown that children as young as 15 months benefit significantly from substantial engagement with their father.
Fathers have a substantial impact on child academic performance.  Studies found that "fathers in two-parent families and nonresident fathers who were moderately or highly involved in their children's school had children who were significantly more likely than children with less involved fathers to receive mostly high marks, enjoy school, and never repeat a grade." 
There is a strong link between a child who is fatherless and criminal activity by that child.  In a study of mostly low-income African-American and Hispanic families, professor Rebekah Levine Coley, found that "Nonresident fathers in low-income, minority families appear to be an important protective factor for adolescents...Greater involvement from fathers may help adolescents develop self-control and self-competence, and may decrease the opportunities adolescents have to engage in problem behaviors." 
"Children with active, involved fathers have better social skills, are healthier, and do better in school", according to Duane Wilson, the Proud Fathers, Proud Parents program coordinator for the Department of Human Services in the State of Michigan.  (2:57)

# Criticisms
Many critics of developmental psychology have noted that studies in the field often fail to account for the effects of genetics. In the book The Nurture Assumption, Judith Harris argues that situational factors in the family environment may not satisfactorily explain observed variation of many traits (such as adult IQ and the Big Five personality factors) in the general population of the United States; rather, Harris suggests that either peer groups or random environmental factors (i.e., those that are independent of family upbringing and socioeconomic status of origin, but not independent of genetics) are more important than family environmental effects  
The book was a 1999 Pulitzer Prize finalist.

# Stages of development

## Prenatal
The prenatal development of human beings is viewed in three separate stages:
- Germinal (conception through week 2)
- Embryonic (weeks 3 through 8)
- Fetal (week 9 through birth)
These stages are not the same as the trimesters of a woman's pregnancy.
The germinal stage begins when a sperm penetrates an egg in the act of conception (normally the result of sexual intercourse between a man and a woman).  At this point a zygote is formed. Through the process of mitosis  the cells divide and double.
The embryonic stage occurs once the zygote has firmly implanted itself into the uterine wall. It is in this stage that the vital organs are formed, and while the external body is still extremely dissimilar from an adult human, some features such as eyes and arms, and eventually ears and feet become recognizable.
The fetal period is when the brain most substantially forms, becoming more and more complex over the last few months.
During pregnancy the risk to the developing child from drugs and other teratogens, spousal abuse and other stress on the mother, nutrition and the age of the mother are quite acute.
Three methods of determining fetal defects and health include the ultrasound, amniocentesis, and chorionic villus sampling.
Ultrasound uses sound waves and a computer monitor, and is non-invasive, thus minimizing potential harm to fetus and mother. Unfortunately its ability to determine potential defect is also far less comprehensive than more risky methods.
Chorionic villus sampling is a form of prenatal diagnosis to determine genetic abnormalities in the fetus. It entails getting a sample of the chorionic villus (placental tissue) and testing it. It is generally carried out only on pregnant women over the age of 35 and those who have a higher risk of Down syndrome and other chromosomal conditions.
The advantage of CVS is that it can be carried out at 10-12 weeks of pregnancy, earlier than amniocentesis (which is carried out at 15-18 weeks). However, it is more risky than amniocentesis, with a 1 in 100 to 200 risk that it will cause a miscarriage.
Amniocentesis is another medical procedure used for prenatal diagnosis, in which a small amount of amniotic fluid is extracted from the amnion around a developing fetus. It is usually offered when there may be an increased risk for genetic conditions (i.e. Down syndrome, sickle-cell disease, cystic fibrosis, etc) in the pregnancy. Amniocentesis done in the second trimester is often said to have a risk of fetal death between about 1 in 400 and 1 in 200. Often, genetic counseling is done before amniocentesis, or other types of genetic testing, is offered.
Although difficult, some methods of treating fetal disorders have been developed, both surgical and drug based. Genetic testing prior to pregnancy is also increasingly available.

## Infancy
From birth until the onset of speech, the child is referred to as an infant. Developmental psychologists vary widely in their assessment of infant psychology, and the influence the outside world has upon it, but certain aspects are relatively clear.
While no agreement has yet been reached regarding the level of stimulation an infant requires, we are well aware that a normal level of stimulation is very important, and that a lack of stimulation and affection can result in retardation and a host of other developmental and social disorders. Some feel that classical music, particularly Mozart is good for an infant's mind. While some tentative research has shown it to be helpful to older children, no conclusive evidence is available involving infants.
The majority of an infant's time is spent in sleep. At first this sleep is evenly spread throughout the day and night, but after a couple of months, infants generally become diurnal.
Infants can be seen to have 6 states, grouped into pairs:
- quiet sleep and active sleep (dreaming, when REM occurs)
- quiet waking, and active waking
- fussing and crying
Infants respond to stimuli differently when in these different states. Habituation is frequently used in testing psychological phenomenon. Both infants and adults look less and less as a result of consistent exposure to a particular stimulus. The amount of time spent looking to a presented alternate stimulus (after habituation to the initial stimulus) is indicative of the strength of the remembered percept of the previous stimulus, or dishabituation.
Habituation is used to discover the resolution of perceptual systems, for example, by habituating a subject to one stimulus, and then observing responses to similar ones, one can detect the smallest degree of difference that is detectable by the subject.
Infants have a wide variety of reflexes, some of which are permanent (blinking, gagging), and others transient in nature. Some with obvious purposes, some are clearly vestigial and some do not have obvious purposes. Primitive reflexes reappear in adults under certain conditions. Namely, neurological conditions like dementia, traumatic lesions, etc. A partial list of infantile reflexes includes:
- Moro reflex or startle reflex:
- Startle
- spreading out the arms (abduction)
- unspreading the arms (adduction)
- Crying (usually)
- Tonic neck reflex or fencer's reflex
- Rooting reflex, sucking reflex, suckling reflex: can be initiated by scratching the infant's cheek; the reaction is pursing of the lips for sucking.
- Stepping reflex, step-up reflex: can be initiated if you support the infant upright from its armpits below a given surface so the baby lifts its foot and steps up on the surface (like climbing a stair).
- Grasp reflex: can be initiated by scratching the infant's palm.
- Parachute reflex: the infant is suspended by the trunk and suddenly lowered as if falling for an instant. The child spontaneously throws out the arms as a protective mechanism. The parachute reflex appears before the onset of walking.
- Plantar reflex or Babinski reflex: a finger is stroked firmly down the outer edge of the baby's sole; the toes spread and extend out.
Infants have significantly worse vision than older children. Infant sight, blurry in earl stages, improves over time. Infants less than 2 months old are thought to be color blind.
Hearing is well-developed prior to birth, however, and a preference for the mother's heartbeat is well established. Infants are fairly good at detecting the direction from which a sound comes, and by 18 months their hearing ability is approximately equal to that of adults.
Smell and taste are present, with infants having been shown to prefer the smell and taste of a banana, while rejecting the taste of shrimp. There is good evidence for infants preferring the smell of their mother to that of others.
Infants have a fully developed sense of touch at birth, and the myth believed by some doctors even today that infants feel no pain is inaccurate. Doctors are slowly becoming aware of the need for pain prevention for newborns.
Piaget felt that there were several sensorimotor stages within his broader Theory of cognitive development.
- The first sub-stage occurs from birth to six weeks and is associated primarily with the development of reflexes. Three primary reflexes are described by Piaget: sucking of objects in the mouth, following moving or interesting objects with the eyes, and closing of the hand when an object makes contact with the palm (palmar grasp). Over these first six weeks of life, these reflexes begin to become voluntary actions; for example, the palmar reflex becomes intentional grasping. (Gruber and Vaneche, 1977).
- The second sub-stage occurs from six weeks to four months and is associated primarily with the development of habits. Primary circular reactions or repeating of an action involving only ones own body begin. An example of this type of reaction would involve something like an infant repeating the motion of passing their hand before their face. Also at this phase, passive reactions, caused by classical or operant conditioning, can begin (Gruber et al., 1977).
- The third sub-stage occurs from four to nine months and is associated primarily with the development of coordination between vision and prehension. Three new abilities occur at this stage: intentional grasping for a desired object, secondary circular reactions, and differentiations between ends and means. At this stage, infants will intentionally grasp the air in the direction of a desired object, often to the amusement of friends and family. Secondary circular reactions, or the repetition of an action involving an external object occur begin; for example, moving a switch to turn on a light repeatedly. The differentiation between means also occurs. This is perhaps one of the most important stages of a child's growth as it signifies the dawn of logic (Gruber et al., 1977). Towards the late part of this sub-stage infants begin to have a sense of object permanence, passing the A-not-B error test.
- The fourth sub-stage occurs from nine to twelve months and is associated primarily with the development of logic and the coordination between means and ends. This is an extremely important stage of development, holding what Piaget calls the "first proper intelligence." Also, this stage marks the beginning of goal orientation, the deliberate planning of steps to meet an objective (Gruber et al. 1977).
- The fifth sub-stage occurs from twelve to eighteen months and is associated primarily with the discovery of new means to meet goals. Piaget describes the child at this juncture as the "young scientist," conducting pseudo-experiments to discover new methods of meeting challenges (Gruber et al. 1977).
- The sixth sub-stage is associated primarily with the beginnings of insight, or true creativity. This marks the passage into the preoperational stage.
When studying infants, the habituation methodology is an example of a method often used to assess their performance. This method allows researchers to obtain information about what types of stimuli an infant is able to discriminate. In this paradigm, infants are habituated to a particular stimulus and are then tested using different stimuli to evaluate discrimination. The critical measure in habituation is the infants' level of interest. Typically, infants prefer stimuli that are novel relative to those they have encountered previously. Several methods are used to measure infants' preference. These include the high-amplitude sucking procedure, in which infants suck on a pacifier more or less depending on their level of interest, the conditioned foot-kick procedure, in which infants move their legs to indicate preference, and the head-turn preference procedure, in which the infant's level of interest is measured by the amount of time spent looking in a particular direction. A key feature of all these methods is that, in each situation, the infant controls the stimuli being presented. This gives researchers a means of measuring discrimination. If an infant is able to discriminate between the habituated stimulus and a novel stimulus, they will show a preference for the novel stimulus. If, however, the infant cannot discriminate between the two stimuli, they will not show a preference for one over the other.
Object permanence is an important stage of cognitive development for infants. Numerous tests regarding it have been done, usually involving a toy, and a crude barrier which is placed in front of the toy, and then removed, repeatedly. In sensorimotor stages 1 and 2, the infant is completely unable to comprehend object permanence. Jean Piaget conducted experiments with infants which led him to conclude that this awareness was typically achieved at eight to nine months of age. Infants before this age are too young to understand object permanence, which explains why infants at this age do not cry when their mothers are gone. "Out of sight, out of mind."  A lack of Object Permanence can lead to A-not-B errors, where children reach for a thing at a place where it should not be. (see also: Infant metaphysics)

## Toddler
Intelligence is demonstrated through the use of symbols, language use matures, and memory and imagination are developed.  Thinking is done in a nonlogical, nonreversible manner.  Egocentric thinking predominates.
Socially, toddlers are little people attempting to become independent. They walk, talk, use the toilet, and get food for themselves. Self-control begins to develop. If taking the initiative to explore, experiment, risk mistakes in trying new things, and test their limits is encouraged by the caretaker(s) the child will become autonomous, self-reliant, and confident. If the caretaker is overprotective or disapproving of independent actions, the toddler may begin to doubt their abilities and feel ashamed for the desire for independence. The child's autonomic development will be inhibited, and be less prepared to successfully deal with the world in the future.

## Early Childhood
When children go to preschool, they broaden their social horizons and become more engaged with those around them. Impulses are channeled into fantasies, which leaves the task of the caretaker to balance eagerness for pursuing adventure, creativity and self expression with the development of responsibility. If caretakers are properly encouraging while being consistently disciplinary, children are more likely to develop positive self-esteem while becoming more responsible, and will follow through on assigned activities. If not allowed to decide which activities to perform, children may begin to feel guilt upon contemplating taking initiative.  This negative association with independence will lead them to let others make decisions in place of them.

## Childhood
In this stage intelligence is demonstrated through logical and systematic manipulation of symbols related to concrete objects.  Operational thinking develops, which means actions are reversible, and egocentric thought diminishes.
Children go through the transition from the world at home to that of school and peers.  Children learn to make things, use tools, and acquire the skills to be a worker and a potential provider.  Children can now receive feedback from outsiders about their accomplishments.  If children can discover pleasure in intellectual stimulation, being productive, seeking success, they will develop a sense of competence. If they are not successful or cannot discover pleasure in the process, they may develop a sense of inferiority and feelings of inadequacy that may haunt them throughout life.  This is when children think of them selves as industrious or as inferior

## Adolescence
Adolescence is the period of life between the onset of puberty and the full commitment to an adult social role, such as worker, parent, and/or citizen. It is the period known for the formation of personal and social identity (see Erik Erikson) and the discovery of moral purpose (see William Damon). Intelligence is demonstrated through the logical use of symbols related to abstract concepts and formal reasoning.  A return to egocentric thought often occurs early in the period.  Only 35% develop the capacity to reason formally during adolescence or adulthood.  (Huitt, W. and Hummel, J. January 1998) 
The adolescent asks "Who am I? Who do I want to be?"  Like toddlers, adolescents must explore, test limits, become autonomous, and commit to an identity, or sense of self.  Different roles, behaviours and ideologies must be tried out to select an identity.  Role confusion and inability to choose vocation can result from a failure to achieve a sense of identity.

## Early Adulthood
The person must learn how to form intimate relationships, both in friendship and love.  The development of this skill relies on the resolution of other stages.  It may be hard to establish intimacy if you haven't developed trust or a sense of identity.  If this skill is not learned the alternative is alienation, isolation, a fear of commitment, and the inability to depend on others.
A related framework for studying this part of the life span is that of Emerging adulthood, introduced in 2000 by Jeffrey Arnett.  Scholars of emerging adulthood are interested not only in relationship development (focusing on the role of dating in helping individuals settle on a long-term spouse/partner), but also the development of sociopolitical views and occupational choice.

## Middle age
Middle adulthood generally refers to the period between ages 40 to 65.  During this period, the middle-aged experience a conflict between generativity and stagnation.  They may either feel a sense of contributing to the next generation and their community or a sense of purposelessness.
Physically, the middle-aged experience a decline in muscular strength, reaction time, sensory keenness, and cardiac output.  Also, women experience menopause and a sharp drop in the hormone estrogen.  Men do not have an equivalent to menopause, but they do experience a decline in sperm count and speed of ejaculation and erection.
Most men and women remain capable of sexual satisfaction after middle age.

## Old age
This stage generally refers to those over 65 years.  During old age, people experience a conflict between integrity vs. despair.  When reflecting on their life, they either feel a sense of accomplishment or failure.
Physically, older people experience a decline in  muscular strength, reaction time, stamina, hearing, distance perception, and the sense of smell.  They also are more susceptible to severe diseases such as cancer and pneumonia due to a weakened immune system.  Mental disintegration may also occur, leading to Dementia or Alzheimer's Disease.  However, partially due to a lifetime's accumulation of antibodies, the elderly are less likely to suffer from common diseases such as the cold or flu.
Whether or not intellectual powers increase or decrease with age remains controversial.  Longitudinal studies have suggested that intellect declines, while cross-sectional studies suggest that intellect is stable. It is generally believed that crystallized intelligence increases up to old age, while fluid intelligence decreases with age.

## Death
see Erikson's stages of psychosocial development

# Schools of psychology

## Cognitive development
Cognitive development is primarily concerned with the ways in which infants and children acquire and develop internal mental capabilities such as problem solving, memory, and language.. Major topics in cognitive development are the study of language acquisition and the development of perceptual and motor skills. Piaget was one of the influential early psychologists to study the development of cognitive abilities. His theory suggests that development proceeds through a set of stages from infancy to adulthood and that there is an end point or goal. Other accounts, such as that of Lev Vygotsky, have suggested that development does not progress through stages rather that the development process that begins at birth and continues until death is too complex for such structure and finality. Rather, from this viewpoint, developmental processes proceed more continuously, thus it should be analyzed, instead of a product to be obtained.
In addition, modern cognitive development has largely moved away from Piagetian stage theories, and is influenced by accounts of domain specificity, which argue that development is guided by innate evolutionarily specificed and content-specific information processing mechanisms.

## Social development
Social psychology is the study of the nature and causes of human social behavior, with an emphasis on how people think towards each other and how they relate to each other. As the mind is the axis around which social behavior pivots, social psychologists tend to study the relationship between mind(s) and social behaviors. In early-modern social science theory, John Stuart Mill, Comte, and others, laid the foundation for social psychology by asserting that human social cognition and behavior could and should be studied scientifically like any other natural science.

## Attachment Theory
Attachment Theory focuses on close, intimate, emotionally meaningful relationships.  Its methods of study involve such approaches as the Strange Situation Protocol developed by Mary Ainsworth and the Adult Attachment Interview developed by Mary Main.  Attachment Theory was developed by Sir John Bowlby.  The attachment is described as a biological system that evolved to ensure the survival of the infant.  Attachment behavior is evoked whenever the person is threatened or stressed and involves actions to move toward the person(s) who create a sense of physical, emotional and psychological safety for the individual. See also the critique by developmental psychology pioneer Jerome Kagan.

# Research methods
Developmental psychology employs many of the research methods used in other areas of psychology. However, infants and children cannot always be tested in the same ways as adults, so different methods are often used to study their development.

## Child research methods
When studying older children, especially adolescents, adult measurements of behavior can often be used, but they may need to be simplified to allow children to perform certain tasks.

## Lifespan development
Developmental psychologists have a number of methods to study changes in individuals over time.
In a longitudinal study, a researcher observes many individuals born at or around the same time (a cohort) and carries out new observations as members of the cohort age. This method can be used to draw conclusions about which types of development are universal (or normative) and occur in most members of a cohort. Researchers may also observe ways in which development varies between individuals and hypothesize about the causes of variation observed in their data. Longitudinal studies often require large amounts of time and funding, making them unfeasible in some situations. Also, because members of a cohort all experience historical events unique to their generation, apparently normative developmental trends may in fact be universal only to their cohort.
In a cross-sectional study, a researcher observes differences between individuals of different ages at the same time. This generally requires less resources than the longitudinal method, and because the individuals come from different cohorts, shared historical events are not so much of a confounding factor. By the same token, however, cross-sectional research may not be the most effective way to study differences between participants, as these differences may result not from their different ages but from their exposure to different historical events.
An accelerated longitudinal design or cross-sequential study combines both methodologies. Here, a researcher observes members of different birth cohorts at the same time, and then tracks all participants over time, charting changes in the groups. By comparing differences and similarities in development, one can more easily determine what changes can be attributed to individual or historical environment, and which are truly universal. Clearly such a study can be even more resource-consuming than a longitudinal study.
Additionally, these are all correlational, not experimental, designs, and so one cannot readily infer causation from the data they yield. Nonetheless, correlational research methods are common in the study of development, in part due to ethical concerns. In a study of the effects of poverty on development, for instance, one cannot easily ralndomly assign certain families to a poverty condition and others to an affluent one, and so observation alone has to suffice.

# Theorists & theories
- Jean Piaget: Theory of cognitive development
- Lawrence Kohlberg: Kohlberg's stages of moral development
- Jerome Bruner: Cognitive (Constructivist) Learning theory / Narrative Construction of Reality
- Lev Vygotsky: Social Contextualism / Zone of Proximal Development
- Urie Bronfenbrenner: Ecological Systems Theory
- Jerome Kagan: one of the key pioneers of developmental psychology
- John Bowlby, Harlow & Harlow, Mary Ainsworth: Attachment theory
- Erik Erikson: Erikson's stages of psychosocial development
- Sigmund Freud: psychosexual development
- Emmy Werner: resilience, risk & protective factors in human development