In general, developmental theories view development as progress from simple to more complex understandings of the self and the world over time. Progress may be continuous in nature, or occurring in stages, but the momentum is most always forward toward greater, more complex understandings. For example, prior to achieving "object permanency", babies do not understand that objects (toys, people) continue to exist even when out of sight. Instead of looking for a toy now covered with a blanket, they instead quickly lose interest in the toy as though it never existed. As they grow, babies come to master the idea of object permanency, and thereafter will begin looking for objects hidden from their view. According to developmental theories, this sort of learning to see the world in increasingly complicated ways continues to occur throughout the lifespan.
Though progress towards development is most certainly happening all the time, the changes that occur are generally gradual in nature. It is only over long periods of time that clear progress from one state to another is apparent. For this reason, developmental theorists tend to view development in terms of stages that people pass through. Each stage is often marked by attainment of a milestone event (such as learning to have object permanence, to walk or talk, or to take responsibility for one's actions).
Due to a phenomena known as "sensitive periods", certain kinds of development can only take place during certain times of life. When development is interrupted during a sensitive period, the normal development that occurs at that time does not occur. Efforts to resume development of affected skills or capabilities after the sensitive period has ended may meet with partial success, but will not typically fully recover what has been lost. Languages are particularly easy for young children to learn, but particularly difficult for older children to learn, for instance. Children who start learning a second language in early childhood, but who stop hearing that language spoken after a while will not pick up that second language. They may thereafter never become fluent in the second language even if they are later exposed to the second language.
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Progress through the developmental stages cannot be forced, but instead needs to occur at its own pace. This is particularly true when speaking about early development, because certain complex mental achievements cannot occur until physical brain development has achieved a certain maturity (brain development continues to evolve long after birth!). Attempts to force a person to achieve a particular developmental stage or state before he or she is ready will typically fail.
Though development cannot be forced to go faster than it wants to go, it can easily become delayed or derailed from further progress. At any given point in a developmental sequence, there are particular challenges that the developing person faces. Wrong turns and choices made at critical junctures or exposure to particularly unnourishing environments can cause children's development to get off track. As developmental progress tends to be cumulative, with success at each new stage in the progression of developmental stages being dependent on earlier success, developmental delays can end up causing serious life problems.
Not all people grow and change at the same rate. Even within a given person, growth can be uneven. A person may grow intellectually into a scholar, but become developmentally delayed with regard to emotional maturity and fail to develop the "emotional intelligence" typical of their peers. This sort of domain-specific developmental delay might occur for any number of reasons, including abuse, drug or alcohol addition, or even the child's overwhelmed reaction to parental conflicts or divorce. Whatever its' cause, such developmentally delayed emotional immaturity could be expected to make that person's life more difficult than it might otherwise have been (filled with loneliness and difficulty forming and maintaining satisfying intimate relationships). In extreme cases, severe developmental delays can result in clinically significant disorders, including mental retardation (when intellectual functioning is severely delayed), certain personality disorders (when emotional coping and maturity is severely delayed), or anger and emotional control problems.
The key insights to take home from developmental theories are that: 1) development is best understood as progress through an ordered series of increasingly complex progressive stages, 2) early developmental delays (interrupting progress towards a particular stage) threaten the developing person's overall progress, and 3) uneven or incomplete development in any important life domain can lead to serious life problems. It is important to identify and correct developmental delays while they are first occurring and as soon as possible, so as to minimize the cumulative damage that delays may cause.