The growth of the cognitive, the physical, the intellectual and the social capacities of the human beings, and their working in normal life, from childhood through old age. It is the subject of the field known as psychology of development. Child psychology was the conventional focus of study, but many people were taught about childhood and adulthood before the mid-20th century. A brief treatment of psychological development follows. See human behavior for more comprehensive therapy.

Infancy 

Infancy occurs one to two years later between birth and language acquisition. In addition, newborns are predestined for certain visual models and certain soons including the human voice, as well as a set of inherited reflexes to help them acquire food and respond to danger.  It is very important to understand human psychology, you can read more about it here

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 and find out where else psychology is used  They can recognize the mothers in the space of a couple of months, showing a strong sensitivity to the tonalities, rhythmic circulation and individual sounds which constitute human speech. Same as older people can coordinate their experiences by making a category for objects and events (e.g. individuals, furniture, food, animals), and even young children are able to make complicated decisions with distance, form, direction, and depth.

Children are rapidly making progress in both recognition and reminder, which in turn improves their understanding and anticipation of events in their surroundings. One fundamental step forward at this time is to recognize the permanence of the object — i.e., to recognize that external objects exist regardless of the perception of them by children. Physical relationships between children and their environment range from simple uncoordinated reflex movements to more co-ordinated activities that are repeated intentional because they are exciting or can be used to accomplish an external objective. When she is around 18 months old, she begins to solve physical problems by imagining certain events and results mentally, instead of by experimenting with a simple trial and mistake.

Childhood

The second phase of human development, childhood, lasts from one or two years until adolescence starts at 12 or 13 years of age. In early infancy the learning and use of language was marked by huge advances. Some months before children actually speak, they begin to understand words. On average, children talk for the first time in 12–14 months and utter about 50 words by the 18th month. Children begin to use two or three word combinations with the use of conjunctions, prepositions, articles and times that go from simple combinations of the noun to sequences that are more grammatically complex. By the fourth year several kids are able to understand in adult sentences and have started to master the more complex grammar and meaning rules.

The children’s cognitive capabilities allow a transition to abstract, symbolic material from relying exclusively on concrete, tangible reality. Even two-year-old toddlers act as if the outside world is a permanent place regardless of their perceptions, and have experimental or goal-oriented behaviour, which can be modified for new purposes creatively and naturally. For the two to seven years children start to use symbolic thinking and language to control the environment and become able to solve logical problems of new types and begin to use versatile and completely reversible mental actions. The beginning of logic between the ages of 7 and 12 are classified in terms of thoughts, a sense of time and number and higher comprehension of seriality and other hierarchical relations.

Emotionally, children are increasingly able to detect and interpret the feelings of others in a way which increases self awareness – that is, the awareness of their own emotional states, characteristics and action potential. This helps to gain empathy or the ability to acknowledge and understand others’ opinions and perceptions. These new skills contribute to the moral growth of children, which generally starts in early years to deal with and prevent acts that cause pain and punishment and to achieve a more general regulation of behavior in order that parental consideration and approval remain.

Adolescence

Age 12 or 13 starts physically at adulthood and ends at the age of 19 or 20 at adult. Adolescence begins with puberty. Intellectually, adolescence is the timeframe in which people will routinely formulate, test, and rationalize hypotheses or suggestions. The formal thought of young people and adults is deductive, sensible and systemic in its own right. Emotionally, teenage life is the moment for people to learn to control and manage their sexual urges and to start to build their own sexual roles.

Adulthood

The adult is a time when people are at their height to respond to the demands for career, marriage and children, all in cultural, emotional and social capacity. In early to mid-adulthood, some psychologists delineate different phases and transitions involving crises or life reassessment that result in decisions about new commitments or objectives. In the mid-30s individuals develop a sense of limited time and prior behavior patterns or beliefs in favor of new ones can be given up.

Middle age is a time of adjustment between historical and future potentialities. In certain people, often referred to as the midlife crisis, an emotional rebellion has occurred because of the acknowledgment that there is less time than was already lived. Dramatic changes in the manufacture of hormones lead to menopause among women. Women often experience the ’empty nest syndrome’ of those children who have grown or left home — feel unwanted or unneeded. Old age, though intelligence tends to lessen sensory and perceptive abilities, muscle strength and memory.