Coming up with a universal explanation for the way humans develop from infant to adult is no easy task. As it stands, there is no one concrete theory but rather a number of different theories which are neither completely right or completely wrong.
The famous philosopher John Locke argued that a child’s mind is a ‘blank slate’ which can be written on and molded whichever way society want it to be. Christian doctrine says that babies are born selfish and must go through a spiritual rebirth where all the evil we are born with is washed from our souls. Philosopher Jean-Jacques Rousseau said that humans are born with an innate goodness and that we susceptible to being spoiled by the evils that exist in the world around us. Though there have been many proposals throughout the ages, everyone has been able to agree on the fact that our minds undergo rapid growth and change as we morph from children to adults. Since philosophy and religion, the field of psychology has been interested in “the scientific study of age-related changes in behavior, thinking, emotions, and personality,”  which is called human development.
Nature vs. Nurture
When discussing human development, a good thing to keep in mind is the classic debate of nature vs. nurture. On one side of the debate we have people arguing that we are born with our personality traits and that our psyche is shaped by purely by internal biological factors like deoxyribonucleic acid (DNA). On the other side there are people arguing that our personalities our molded purely by external factors such as family, friends, teachers, and society as a whole. The general modern scientific consensus on the debate is that our personalities are not shaped entirely by one or the other but rather by a mix of both biological and environmental factors. Most modern day psychologists disregard the debate as being outdated.  Most professionals acknowledge that we are born with a temperament that will follow us our throughout our lives. Just because someone is born with an aggressive/fussy temperament does not mean that they will grow up to commit acts of violence, though it helps. At the same time, someone with a docile temperament might be capable of murder. What is certain is that our heredity and DNA can carry traits which make us susceptible to mental disorders and psychopathology, which can have serious affects on personality. 
Scientists studying human development divide factors of growth into three different categories called the domains of development.
- Physical Domain - The physiological processes and changes that occur in the human body such as puberty.
- Cognitive Domain – Changes to the way we think, our intelligence, memory capabilities, etc.
- Social Domain – Changes that occur between an individual and other people, including development of social skills, high self esteem.
The domains often overlap as people go through developmental changes. For instance, the onset of clinical depression not only changes the way someone thinks but can also disturb their social lives and lead to physiological changes such as weight gain or loss.
- Quantitative Change – As a child gets older they may go from having zero friends to having ten, or from 3 foot 2 inches to 6 foot 3 inches. These changes in numbers are classified as quantitative change. This is sometimes referred to as continuity.
- Qualitative Change - If a child goes from enjoying Pokemon card collecting to football card collecting it is change in kind or type, which makes it a qualitative change. Changes in likes, dislikes, and actions are sometimes called discontinuity.
- Social Clock – Each culture has a series of changes they expect every human to go through as they age. This sequence of changes and experiences is called the social clock.
- Critical Period – When a human is sensitive to the presence or absence of some kind of experience, such as not being able to go to their high school prom.
- Atypical Development - If a person changes in unexpected ways that are not typical and it is ultimately harmful to the individual then they have experiences atypical development.
To support their theories or hypotheses, Psychologists will do research and perform tests to obtain empirical evidence that either backs up or discredits their claims. When dealing with human development, these methods are often used for age-related studies. There are three commonly used research method designs.
Cross-sectional Design – When groups of people of different age are compared to one another, a cross-sectional design is used. This is helpful when trying to see how something changes in individuals during different stages of their lives. For instance, a cross-sectional designed study might take three people in their 20′s, three in their 30′s, and three in their 40′s. Then they test their memory capabilities to decide whether or not memory declines as we age.
Longitudinal Designs – The problem with cross-sectional design is that it takes random individuals and studies them for only a short period of time, which does not really show how each individual changes as they age. With longitudinal designs, the same people will be studied over a long period of time. This helps find errors in the study. For instance, a person might show good memory when they are 20 years old and still have just as good a memory when they are 60. Since their memory did not decline it might mean the hypothesis is flawed.
Sequential Designs - Longitudinal designs have their problems too. They involve getting the same people over and over again to take the same tests or do the same things. People might learn how to perfect their scores on these tests and others might decide they don’t want to help with the study anymore. Sequential designs mix aspects of cross-sectional and longitudinal designs to try and reduce the errors that occur in such studies. People are studied over time but cohorts are used. A cohort is someone who has experienced the same things as someone else and are the same age. This helps to show whether or not results are consistent in everyone.
When a psychologist does a very detailed examination of one individual it is called a case study. This is helpful when trying to determine why or how someone is experiencing a stage or aspect of life. Humans are very complicated and individualistic creatures and case studies help to hone in on one individual’s uniqueness and solve their specific problems or shed insight on humans the way large scale studies cannot. 
During the course of a case study, a psychologist might use naturalistic observation, which is the process of studying people in their normal environment. This can offer much more insight into the way humans think and are affected by society. It can also lead to better data since the subject does not feel awkward or out of place due to a fabricated environment.
The most famous psychologist of all time, Sigmund Freud, was the founder of psychoanalysis and psychoanalytic theories.  These theories are based on the belief that developmental changes occur because of the influence of internal drives and emotions on behavior.
Freud’s Psychosexual Theory
From Freud’s psychoanalysis theories psychologists were given one possible insight as to how the conscious and unconscious mind work.  Freud believed that there is a constant unconscious drive in humans to seek pleasure, which he called the libido. He argued that the human personality can be divided into three different parts.
- id – Part of the unconscious, the id houses the libido. It is the instinct all humans are born with that gives us carnal, animal like desire. It causes us to pursue sex and be aggressive.
- ego – The ego develops in our first 2 to 3 years of life and is the conscious, thinking part of our minds. The ego is the part of our personalities we are most aware of. Part of the ego’s job is to help satisfy the needs of the id. It’s job is to keep all three components of personality in harmony.
- superego – The superego is the root of our morality. The rules we are taught by family, friends, and society all merge and become part of the superego. The superego helps us decide between right and wrong. The ego’s job is to try and satisfy the id without going against rules set forth by the superego.
When these components are in conflict with one another it creates tension and emotional problems. To get rid of this tension a person might use defense mechanisms which are ways of thinking about a situation to reduce anxiety. Examples of defense mechanisms include repression, denial, or projection.  Without defense mechanisms, Freud believed that the person with conflicting personality components would be under so much stress that they develop mental illness or kill themselves.
While Freud was first psychoanalyzing his patients he noticed a trend. Almost all of them had memories of sexual feelings and behaviors from their childhood. As a result, Freud developed a series of stages that he believed everyone went through. These stages all had to do with sexual development. If a child was not given an upbringing which successfully carried them through these stages then they would develop a fixation. They become study at a certain stage, which results in the person acting in ways that show unresolved problems and unmet needs.
The Oedipus complex is a collection of the feelings a young boy has during their phallic stage. According to Freud, boys this age are supposed to have very strong feelings for their mother and because of those feelings they develop a jealousy toward their father. Sometimes the feelings are so strong that they want to kill their father. Girls will have a similar experience called the elektra complex, where their attraction to their father will result in feelings of distaste for their mothers.
Erikson’s Psychosocial Stages
Erik Erikson was a psychologist who had an alternative opinion to Freud’s about human development. Erikson proposed a theory composed of various stages but rather than focusing on sex he focused on social changes. His theory states that we go through eight stages and that in each one we face a conflict that can either be successfully or unsuccessfully resolved.
The positive characteristics shown in the diagram will only be gained if the dilemma in the stage is successfully resolved. In the case of trust vs. mistrust, the dilemma should be resolved so that the person has trust rather than mistrust.
Freud’s psychosexual theory has some shortcomings in that sexual feelings are probably not as important to development as Freud believed. Erikson’s psychosocial theory has some shortcomings in that it is very simplified.
Whereas psychoanalytic focus on internal drives and emotions, learning theories propose that through external factors, especially the environment, people can be trained and learn to do or be anything. John Watson, the psychologist who first developed learning theories , called this behavioralism.  Behavioralism defines development in terms of behavior changes caused by environment influences. Learning theories say that development should happen through the accumulation experiences.
Pavlov’s Classical Conditioning
Ivan Pavlov found that humans can acquire new signals for existing behaviors. This means that we learn through stimuli. We come to expect certain stimulating effects from certain things. He saw that when we know something should happen when we develop a conditioned response. 
Skinner’s Operant Conditioning
B.F. Skinner proposed operant conditioning, which says that people can learn to repeat or stop behaviors because of the consequences that go along with them. Reinforcement is anything that follows a behavior and causes it to be repeated. This can be as simple as giving out a piece of candy every time the
subject has successfully done something. Positive reinforcement would be something pleasant which causes the subject to repeat the behavior over again. It reinforces the action and
causes them to do it again in hopes of recieving candy. Punishment is anything that follows a behavior and causes it to stop. Also called negative reinforcement or nonreinforcement, this might involve taking an insulin shot to prevent the negative effects of diabetes. The shot isn’t pleasant but most be done to prevent a less pleasant occurence. Extinction occurs when a behavior is completely eliminated through nonreinforcement.
Shaping is the learning of complex behavior through the reinforcement of intermediate steps. An example of this would be learning to play a sport. You learn one step at a time, reinforcing yourself alont the way, until you can finally bring all of the steps together to successfully play the sport.
Bandura’s Social-Learning Theory
Albert Bandura was a psychologist who had the idea that learning does not necessarilty require reinforcement. He believed that we can also learn by observing other people. Children learn how to behave by looking at their siblings and other adults. Bandura believes that this kind of learning depends on four things.
- What the person pays attention to
- What they are able to remember
- What they are physicially able to copy
- What they are motivated to imitate.
Bandura also believed that we learn from not only behavior but also ideas, expectations, internal standards, and self-concepts. Bandura calls this self-efficacy which can influence our well-being and physical health as well as learning.
When theories emphasize mental aspects of development such as memory and logic then they are categorized as cognitive theories. These theories look at the human mind as the focal point and base everything around that.
Piaget’s Cognitive Developmental Theory
Jean Piaget developed a very famous theory about human development. There are a number of terms that accompaned Piaget’s influential theory.
- scheme – An internal cognitive structure that provides an individual with a procedure to use in a specific circumstance.
- assimilation – When we use scemes to make sense of experiences it is called assimilation.
- accomodation – Schemes are sometimes changed as a result of some new information acquired through assimilation.
- equilibrium – The process of balancing assimilation and accomodation to create schemas that fit the environment someone is in.
Piaget divided his theory into stages:
Information-processing Theory – A theoretical model that uses the computer as a model to explain how the mind manages information. Often this model will show how humans receive incoming sensory data which is stored in sensory memory, then sent to short term memory which can then be sent into long-term memory.  Since Piaget’s theory there has been the development of neo-Piagetian theory. Neo-Piagetian theories often use information-processing principles to explain the development stages in Piaget’s theory.
Theories that combine physiological and biological aspects of development are called biological theories. Some can address groups of people and others focus on individuals.There are certain beliefs that biological theories will follow or exhibit, although they are not required.
- Nativism – There is a belief that human beings have genetic traits that will be exhibited in all members of the species. This is called nativism, and is used to explain how people are programmed to be a certain way by their genetic code which the environment has no influence on.
- Ethology – Ethologists believe that survival behaviors evolved through natural selection. Behaviors and skills that are successful will be passed on to the next generation. Those that do not have these necessary skills will die and not pass on their genes. Ethologists also believe that emotional relationships are required for the survival of human infants. For instance, if a mother didn’t emotionally respond the the cries of her child then she would have no desire to take care of that child.
- Sociobiology - The study of society using the methods and concepts of biology is called sociobiology. Genes that help groups of people survive are often the core interest of sociobiologists. Sociobiology tries to find the common links in all of human society, or the universal rules that all humans seem biologically programmed to create. Evolution selects the genes that will best promote the survival of the human species.
- Behavior Genetics – Studying how heredity affects individual differences among humans is called behavior genetics. Heredity has been show to affect such things as intelligence, aggressiveness, and shyness. Behavior geneticists have studied identical twins to see what characteristics are present in both because they have the same DNA.
Vgotsky’s Sociocultural Theory
Piaget believed that complex thinking were developed through a child’s explorations. Vgotsky proposed that complex thinking develops through social interactions.  When a child’s learning experience is guided by an adult it is called scaffolding. For successful scaffolding an adult must model the best strategy, gain and keep the child’s attention, and adapt the learning to the child’s developmental level, which Vgotsky called the zone of proximal development.
Bronfenbrenner’s Ecological Theory
Who we are and the actions we make are not defined solely by our inner cognitive mechanism, but are decided in large by exterior environment. Such is the theory presented by scholar Urie Bronfenbrenner in his Ecological Systems Theory. Though Bronfenbrenner has passed away, his work is still widely admired today and was undoubtedly a leader in the field of developmental psychology. His Ecological Systems Theory was first published in 1979. In this theory, Bronfenbrenner presents the concept that there are four environmental systems interlinked bi-directionally that each play a specific role in the development of humans throughout their lifespan. These systems are the microsystem, the exosystem, and the macrosystem. 
Diagram Courtesy of http://reginaldwilliams.blogspot.com/2007_04_24_archive.html
A modern trend in human developmental studies is the use of many theories and perspectives to explain and study development. This is called eclecticism and it is helpful because psychologists are no longer restricted to the rigid confines of just one theory anymore. If something isn’t accounted for in one theory, then the psychologist can take it from another theory to help with his or her studies. In a sense the theories are molded to represent a more realistic view of human kind.
All of the theories discussed so far have their own merit and usefulness. A developmentalist would look at all of these theories and apply the one which fits their current purposes best. To do this, it is helpful to compare all of the theories at once:
None of these theories are completely correct and none of them are incorrect. They are proposals that shed light on the development of human beings. Psychologists will always argue over which theories are superior to others but the most important thing to keep in mind it doesn’t matter. If a theory helps you do your work then it has served its purpose. If you disagree with any of the theories then you can modify them or create your own. Human development is a very interesting and important focus of study and its theories will continue to be useful for anyone interested in the biological and psychological sciences.
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- Case Studies. Writing@CSU Home Page
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- FREUD's Theory of Psychosexual Development. a2zpsychology.com
- Section 6: Freud's Ego Defense Mechanisms. AllPsych Online.
- Classical Conditioning. Ivan Pavlov
- Social Learning Theory. Ormrod, J.E. (1999). Human learning (3rd ed.). Upper Saddle River, NJ: Prentice-Hall.
- Theories Used in Research Information Processing Theory (2005). Appalachian State University
- Vygotsky & Socio-Cultural Theory. KidsDevelopment
- Bronfenbrenner, U. (1979). The ecology of human development. Cambridge, MA: Harvard University Press.