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 on 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.