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[[Image:Hans Baldung 009.jpg||thumb|200px|[[Hans Baldung Grien]]: The Ages And [[Death]], c. 1540-1543]]'''[[Developmental ]] psychology''' is the [[science|scientific]] study of progressive [[psychology|psychological]] changes that occur in [[human]] beings as they age. Originally concerned with [[infants]] and [[children]], and later [[other ]] periods of great [[change ]] such as [[adolescence]] and [[aging]], it now encompasses 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]], acquisition of [[language]], [[morality|moral understanding]], and [[identity (social science)|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. Other issues that they deal with is the question of whether [[development ]] occurs through the gradual accumulation of [[knowledge]] or through shifts from one [[stage ]] of [[thinking]] to [[another]]; or if children are [[born ]] with innate knowledge or [[figure ]] things out through [[experience]]; and whether development driven by the [[social context]] or by something [[inside ]] each [[child]].

Many [[theoretical ]] perspectives attempt to explain development, among the most prominent are: [[Jean Piaget]]'s [[Theory of cognitive development|Stage Theory]], [[Lev Vygotsky]]'s [[Social Contextualism]] (and its heir, the Development in Context or Human [[Ecology ]] [[theory ]] of [[Urie Bronfenbrenner]]), and especially the [[information processing]] framework employed by cognitive psychology.

Historical theories continue to provide a basis for additional research, among [[them ]] are [[Erik Erikson]]'s eight [[Erikson's stages of psychosocial development|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 [[Kohlberg's stages of moral development|stages in moral reasoning]].

Generally regarded as one of the [[world]]'s leading scholars in the field of [[developmental psychology]], Bronfenbrenner's primary contribution was his [[Ecological Systems Theory]], in which he delineated four types of nested systems, with bi-directional influences within and between systems.

#Mesosystem: A [[system ]] comprised of connections between immediate environments (i.e., a child’s home and school)#Exosystem: [[External ]] environmental settings which only indirectly [[affect ]] development (such as parent's workplace)#Macrosystem: The larger [[Culture|cultural]] context ([[Eastern culture|Eastern]] vs. [[Western culture]], national [[economy]], [[subculture]])

The person's own [[biology ]] is considered part of the microsystem; thus the theory has recently sometimes been called Bio-Ecological Systems Theory.

Each system contains roles, norms and rules that can powerfully shape development. For example, an inner-city black family faces many challenges which an affluent white family in a gated [[community ]] does not, and vice versa. The inner-city black family is more likely to experience environmental hardships, such as [[teratogen]]s and crime. The sheltered white family on the other hand is more likely to lack the nurturing support of extended family.

The major [[statement ]] of this theory, ''The ecology of human development'' (1979) had widespread influence on the way psychologists and [[others ]] approached the study of human beings and their environments. It has been said that before Bronfenbrenner, child psychologists studied the child, sociologists examined the family, anthropologists the [[society]], economists the [[economic ]] framework of the [[times ]] and [[political ]] scientists the [[structure]]. As a result of Bronfenbrenner's groundbreaking [[work ]] in "human ecology," a field that he created, these environments - from the family to economic and political [[structures ]] - were viewed as part of the life course from [[childhood ]] to [[adulthood]].

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 [[modularity of mind|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 nativist position on this issue, [[radical behaviorism|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.

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 [[model (abstract)|models]]. Developmental models are often [[computer model|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]], [[connectionism|connectionist]] ([[neural net|neural network]]), or [[dynamical system]]s models to explain the mechanisms of development.

The modern [[form ]] of developmental psychology has its roots in the rich psychological [[tradition ]] represented by [[Heraclitus]], [[Aristotle]] and [[Descartes]]. [[William Shakespeare]] had his [[melancholy ]] [[character ]] ''Jacques'' (in [[As You Like It]]) articulate the [http://www.ac.wwu.edu/~stephan/webstuff/poetry/Shakespeare-TheSeven.html ''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.

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 development|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''; his descriptions have been taken up by educators (in the [[Waldorf Schools]]) and by psychologists (in [[biographical therapy]]; see the works of [[Bernard Lievegoed]]). By the early to mid-[[twentieth century]], the work of [[Vygotsky]] and [[Piaget]], mentioned above, had established a strong [[empirical]] tradition in the field.

Traditionally mothers (and [[women ]] generally) were emphasized to the [[exclusion ]] of other caretakers. This has begun to change, with the emphasis now placed on a [[primary caretaker]] (regardless of [[gender ]] or [[biological ]] relation), as well as all persons directly or indirectly influencing the child (the family system).

For example, the traditional [[father ]] had little to do with an infant ''directly'', but his method of interacting with the [[mother ]] (supportive, abusive, neglectful) had a great deal of impact on the infant ''indirectly''.

The prenatal development of human beings is viewed in three [[separate ]] stages:

#[[Fetal]] (week 9 through [[birth]])

The germinal stage least resembles a grown human. It 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.

[[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]] ([[placenta]]l 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 [[medicine|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.

From birth until the child begins to [[speak]], they are referred to as an [[infant]]. Developmental psychologists vary widely in their assessment of the infants 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 [[European classical music|classical music]], particularly [[Wolfgang Amadeus Mozart|Mozart]] is [[good ]] for an infants [[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 infants time is spent in [[sleep]]. At first this sleep is evenly spread throughout the day and night, but after a couple of months they generally become [[diurnal]].

*quiet sleep and [[active ]] sleep ([[dreaming]], when [[REM]] occurs)

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 vestigal 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:

*[[Stepping reflex]], [[step-up reflex]]: it 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).

Infants have particularly poor [[vision]], and are [[legally blind]]. They are capable of [[sight]], however blurry. This improves over time, based on experience. Infants less than 2 months old are also [[thought ]] to be [[color blind]].

[[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.

*The first sub-stage occurs from birth to six weeks and is associated primarily with the development of [[reflex]]es. 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 [[habit]]s. 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 conditioning|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 [[Visual perception|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 (trait)|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 infants 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 sersorimotor 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 error]]s, where children reach for a [[thing ]] at a place where it should not be.

Cognitive development is primarily concerned with the ways in which infants and children acquire cognitive abilities. 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’s 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.

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]].

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 (statistics)|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.

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 [[correlation]]al, not [[experiment]]al, designs, and so one cannot readily infer [[causality|causation]] from the data they yield. Nonetheless, correlational research methods are common in the study of development, in part due to [[Research ethics|ethical]] concerns. In a study of the effects of poverty on development, for [[instance]], one cannot easilly [[random assignment|randomly assign]] certain families to a poverty condition and others to an affluent one, and so observation alone has to suffice.