Increasing age, by itself, contributes nothing to development. The maturation and changes resulting from experience that intervene between different ages and stages of childhood are important. Maturation aspects are aspects of development that are largely under genetic control and hence largely uninfluenced by environmental factors.

Folk theories of development are ideas held about development that is not based upon scientific investigation There are two main folk theories:

1. Praise
   Children are born inherently good and physical punishment is not necessary.
2. Punishment
   This theory bases itself on the idea that a child is born in sin and his soul should be saved and punishment is necessary for this. Punishment is necessary regularly in order to develop as pleasant, law-abiding citizens.

A paradigm is a world view or a world hypothesis. There are two main paradigms in many developmental theories:

1. Organismic world view
   This is the idea that people are inherently active and continually interacting with the environment and therefore helping shape their own development. An example of this world view is Piaget’s theory. There is no returning to former stages of development.
2. Mechanistic world view
   This is the idea that a person can be represented as being like a machine, which is inherently passive until stimulated by the environment. It is possible to return to former stages of development as the frequency of behaviour can increase and decrease due to various learning processes.

There are three main ways of studying age-related changes:

The results of the cross-sectional design and the longitudinal design can differ because of two effects:

1. Time between measures
   If different distances between test ages are used, it can result in very different developmental function. Some developmental functions are not revealed unless frequent measures are taken on individuals (e.g: growing a centimetre overnight).
2. Cohort effects
   There are changes across generations in the characteristic one is interested in. This can give different results (e.g: if a 4-year-old is simultaneously tested with a 16-year-old, the data can differ, because the procedures and the measures have changed in 12 years’ time).  It occurs if other factors than the research characteristic changed over time.

There are several research methods used in order to investigate development. Observational studies are studies in which the behaviour is observed and recorded, but the research does not attempt to influence the behaviour in any way. Case studies are an example of observational studies, as one individual is measured multiple times over time. Baby biographies are diaries detailing an infant’s development. The strengths of baby biographies are that it can lead to the production of theories of child development and that the biographer can give a detailed account of subtle changes in behaviour because of their intimate knowledge of the child.

Time sampling is an observational method that records an individual’s behaviour at frequent intervals of time. Two criticisms of time sampling are that the observer might not get an accurate record of the amount of time spent in different behaviours and that some behaviours might be missed. Event sampling is an alternative method that avoids these problems. One or multiple events are selected and this event is recorded throughout its time period.

The clinical method is a research method whereby natural behaviour is observed and then the individual’s environment is changed in order to understand the behaviour of interest better. Infants grasp pictures of objects because the objects can be too enticing for them. Structured observation is an observational study in which the independent variable is systematically controlled and varied and the investigator then observes the child’s behaviour. The degree is less than in an experimental setting.

Psychological tests are instruments for the quantitative assessment of some psychological attribute or attributes of a person. These tests are usually standardised on large samples of children of the appropriate age groups. The type of test items included in a particular test will depend both on the age group and what is being measured. Personality traits are difficult to measure and are not always good predictors in infancy for adulthood, although aggression, shyness and boldness tend to stay the time.

There are two useful types of correlational studies for developmental psychology:

1. Concurrent correlational studies
   This is a study where a relationship between variables that are measured at the same time is the interest (e.g: IQ differences between twins).
2. Predictive correlational studies
   This is a study where we are interested in finding whether individuals retain their relative standing, or rank order, relative to others, over time.

There are several methods for neurodevelopmental studies. The marker tasks are a method designed to elicit a behaviour with a known neural basis. It is used to find analogous brain areas between animals and humans. There are several imaging methods for brain activity in humans:

1. EEG
   A scalp recording was done with an electrode that measures electrical activity produced by neurons. It is difficult to find a specific brain area with this method.
2. ERP
   A scalp recording in which brain activity is monitored during the presentation of specific perceptual events. It is difficult to find a specific brain area with this method.
3. PET
   An imaging method measuring cortical activity. Blood flow to tissues in the body is measured. This requires the injection of a short-lived radioactive isotope.
4. fMRI
   An imaging method measuring cortical activity which works by measuring blood flow.

The catharsis hypothesis is the argument that watching aggressive tendencies in others will reduce your own feelings of aggression, but this argument is false. There are many examples of folk theories being proven wrong with systematic observations or experiments. Developmental research has also implications in social policies, as is shown by the Head Start programme in the USA and the Sure Start programme in the UK.

Developmental functions are typical trends in development (e.g: we get more intelligent as we age). Developmental functions are presented in graphs and they can be used to detect unusual patterns of behaviour. There are different types of graphs:

1. Continuous function – increasing ability
   This is a type of behaviour that improves with age, for example, the precision to grab objects.
2. Continuous function – decreasing ability
   This is a type of behaviour that worsens with age, such as speech perception (e.g: infants can hear the difference in speech between different language and later starts to discriminate).
3. Discontinuous (step) function
   This is a type of behaviour where development takes place in steps or stages. Each stage appears to be qualitatively different from the preceding and following stages (e.g: infancy and puberty).
4. U-shaped functions
   This is behaviour where the ability is initially very good, then decreases and then increases again. An inverted U-shaped function has the opposite pattern. An example of the U-shaped function is making stepping movements (which infants are able to make) and an example of the inverted U-shaped function is visual acuity.