Developmental psychology is a sub-discipline of psychology which is generally concerned with human development, both physical and cognitive. It studies the age-related changes through the various stages in a person’s life like their physical, mental, emotional, intellectual, and social changes. However, it should be mentioned that most of the psychologists who refer to themselves as developmental psychologists are interested in childhood, so much so that for many the term developmental psychology has become identical to child psychology. The past focus was on child development but in the past 25 years researchers who study human development have expanded their focus to include the study of the physical, motor, cognitive, intellectual, emotional, personality, social, and moral changes within a person’s lifespan. While many things in the field of developmental psychology encompass childhood, it isn’t exclusively about children with some research delving into adult development. Nevertheless, it is true to say that most of the research within developmental psychology concerns infants, children or adolescents. Beginning with Sigmund Freud and Jean Piaget in the early part of the 20th century, two main methods in psychology examined the psychological development of humans from childhood to adulthood. The psychoanalytic theory of Freud gave a description of psychosexual development in children, and behaviorism described the mechanics of the learning process. Until the 1930s, the study of development concerning the psychological, emotional, and perceptual changes that happen during an individual’s lifetime did not evolve until Piaget invalidated traditional thinking with the idea that a child is not just a “miniature adult” obtaining knowledge as his or her body matures, but at the same time is also going through rigorous psychological changes. One of the foremost figures in developmental psychology is Jean Piaget who argued that all humans develop through a similar route, advancing through distinguishable “stages”, each with known characteristics and psycho-social goals that must be fulfilled if one is to progress to the next stage. Piaget proposed four stages: the sensorimotor, preoperational, concrete operational, and formal operational. The first stage, sensorimotor, takes place from birth to age 2 and is distinguished by the idea that infants “think” by influencing the world around them. This is accomplished by using all five senses such as hearing, seeing, touching, tasting, and smelling. The main achievement during this stage is object permanence which is knowing that an object still exists, even if it is not shown to them directly. An example of this accomplished goal is when the child learns that even though his parents have left the room, they have not ceased to exist. Near the end of this stage, children can engage in “deferred imitation”, a term coined by Piaget. This deferred imitation is the ability to replicate or repeat a previously witnessed action later in the future as opposed to copying it right away. Next is the preoperational stage which happens from age 2 to age 7 and in this stage children can use signs to represent words, images, and ideas alongside being able to think about things symbolically. The children in this stage take part in their imagination pretending to be various things such as a child pretending to be an airplane and having their arms up simulating the wings of the plane. However, the children in this stage can not yet logically think meaning they cannot rationalize or understand more complex ideas just yet. Following this stage is the concrete operational stage which takes place from age 7 to age 11 and is generally characterized by the conception that children’s reasoning becomes more focused and logical. Children are able to show a logical understanding of conservation principles which is the ability to recognize that key properties of a substance do not change even as their physical appearance may be altered. For example, a child will be able distinguish identical amounts of liquid and understand that they will remain the same despite the size of the container in which they have been poured. The last stage in this theory is the formal operational stage which occurs from age 11 to adulthood. It is defined by the idea that children develop the ability to think in abstract ways. This allows children to participate in the problem-solving method of developing a hypothesis and reasoning their way to plausible solutions. Thinking is no longer tied to events that individuals observe and they make use of logic to resolve problems. Sigmund Freud was one of the most influential figures of the twentieth century and the inventor of psychoanalysis. While some parts of his theories remain controversial, much of his work has become such a fundamental part of our culture that it is taken for granted. Freud believed that the main areas of instinctual gratification and the erogenous zones moved across childhood in predictable stages. Freud developed the psychosexual theory that suggested that children develop through a series of stages related to the erogenous zones. His psychosexual development theory included five stages called oral, anal, phallic, latency, and genital. Each psychosexual stage had specific psychological characteristics to it. According to this theory, each stage of development must be fulfilled for proper development and if we lack proper nurturing and parenting during a stage, we may become fixated on that stage. The oral stage which begins from birth to age one is when the child derives pleasure from oral activities, such as sucking and tasting. Successful fulfillment of the infant’s feeding need’s and proper weaning may result in the establishment of trust between child and parent. The anal stage takes place between ages one and three and the main source of gratification is the ability to control bladder movement and the elimination or retention of feces. The control they learn to exert over their bodily functions is manifested in toilet-training. They learn to exert their bodily functions and toilet-training is manifested in. Improper resolution of this stage such as the parent’s toilet training their children too early can result in a child who is uptight and overly obsessed with order. Than there’s the phallic stage which takes place between three to six years of age. According to Freud, it’s during this stage that preschoolers take pleasure in their genitals and begin to struggle with sexual desires toward the opposite sex parent. For a male boy this is called the Oedipus complex which involves a boy’s desire for his mother and his urge to replace his father who is seen as a rival for the mother’s attention. Than for the girl’s it’s called the Electra complex, which was actually later proposed by Freud’s protégé Carl Jung and involves a girl’s desire for her father’s attention and wish to take her mother’s place. Latency happens six to twelve years of age and in this stage, sexual instincts subside, and children begin to further develop the superego, or conscience. The individual spends most of his time interacting with same sex peers, engaging in hobbies and acquiring skills. After the age of twelve and onward is the genital stage and during this stage sexual impulses reemerge. If other stages have been successfully met, adolescents engage in appropriate sexual behavior which may lead to marriage and childbirth. Sigmund Freud’s Theory of Psychosexual Development is one of the most complex and controversial theories of development. Even though his theory has been the subject of much criticism, we cannot overlook the important ideas that Freud has contributed to developmental psychology.Another psychologist who has been very influential is Erik Erikson, he focused on the entire lifespan of humans and actually built off the theory’s proposed by one of psychology’s most influential researchers, Sigmund Freud. Erikson took Freud’s controversial psychosexual theory and modified it into an eight-stage psychosocial theory of development. Freud’s initial theory had five stages, but Erikson added three more stages onto that. Unlike Freud, Erikson thought that lack of success at any given stage is not disastrous for the whole of personality development. In case of failure the dilemma of the stage can be resolved during the next period, but accomplishing this takes much more effort. These stages proposed by Erikson are trust vs. mistrust, autonomy vs. shame/doubt, initiative vs. guilt, industry vs. inferiority, identity vs. role confusion, intimacy vs. isolation, generativity vs. stagnation, and integrity vs. despair. Erikson proposed that we are encouraged by the need to achieve capability and proficiency in certain aspects of our lives. According to Erikson’s psychosocial theory, we experience eight stages of development over our lifespan, from infancy through late adulthood. At each stage there is a crisis or task that we need to resolve. Whenever we experience such crisis, we are left with no choice but to face it and think of ways to solve it. Failure to overcome such dilemmas may lead to significant impact on our psychosocial development. Successful completion of each developmental task results in a sense of competence and a healthy personality. Failure to master these tasks leads to feelings of inadequacy. While some theories have similar or contradicting views, most scholars today agree that psychological development happens over a life span and that developmental psychology is not confined to the study of any particular stage of life. These psychologists have developed many theories on developmental psychology and while some have proven to be not exactly true, these theories help to lay the basics on understanding human psychological development.