Developmental delay refers to a delayed, but normal path of development. A developmental difference refers to a qualitatively different path of development. Whether the development is delayed or different depends on the area of development. One approach to quantifying a delay includes looking at the extent to which individual children perform relative to a level expected for their chronological age on standardised assessment tests. Spotting atypical development can also be done by checking the scores of a test of children and comparing them with the population. This makes use of standard deviations. Concluding that one aspect of development is delayed doesn’t tell us anything about what underlies the delay.

The study of atypically developing children provides a profile of the main behaviours associated with a condition within the context of development across the human lifespan. Atypical trajectory refers to a sequence of development that departs from the typical sequence. The study of atypical development can result in effective interventions and it can also teach us something about typical development.

A conventional methodological method is making a comparison between the performance of the atypical sample and the performance of the relevant control group sample. It is common to compare a clinical group with two control groups, using a standardised test. By checking the difference of the clinical group to the mental age group and the chronological age group, it is possible to determine whether the clinical group has a delay or a qualitatively different development.

The human genome project found that there were fewer genes than previously thought and this is a strong indicator that there was more to specifying humanity than the action of individual genes in isolation (e.g: not one gene causes disease because there aren’t enough genes for that). The study showed that there is no one-to-one mapping between a DNA gene to a specific protein and an associated inherited trait. The complex interaction between genes leads to traits and not a single gene. This project changed the way we look at atypical development.

Neuroimaging tools aid localisation of brain activity that enables the developmental psychologist to understand more about the pathways associated with atypical development. Neuroimaging tools that are typically used in order to research development are PET, ERP and MEG. These tools make sure that we know more about the brain areas and processing speeds related to atypical development compared to typical development.

Eye-tracking technology allows precise measures of visual behaviour. One advantage of eye-tracking measurements is that it takes no explicit verbal instructions and can thus also be used for infants.

Atypical conditions of childhood can be characterised according to the type of causal pathways involved. There are specific genetic conditions, but there are also conditions that don’t have a known specific genetic defect (e.g: ADHD).

Williams Syndrome is an extremely rare condition. This makes it difficult to conduct research about this syndrome. People with Williams syndrome have low non-verbal IQ, difficulties in planning, problem-solving and spatial cognition. They are highly sociable and communicative. Pragmatics refers to the competence of speakers to communicate in socially appropriate ways. People with Williams syndrome have difficulties with pragmatics. Their speech is overly dependent on the adult’s leads and contributions, with difficulties in leading the conversation forwards reflected in the ability to add new information and respond to requests for new information.

There are no reliable biological indicators, such as brain abnormalities, that can diagnose autism. Autism can be depicted according to the triad of atypical behaviours. One behaviour associated with impairments in social interaction is atypical eye contact. There are three main theories that account for the three main deficits associated with autism:

1. Theory of mind hypothesis
   This is the view that people with autism have difficulties in understanding that others have thoughts and beliefs. The capacity to understand mental states do not necessarily coincide with social functioning.
2. Executive function hypothesis
   This is the view that autism is due to a deficit in executive function. Research has shown that brain areas related to executive functioning are used abnormally by people with autism.
3. Central coherence hypothesis
   This is the view that autism is due to a lack of central coherence, which is reflected in the tendency to process information piecemeal rather than to integrate it.

People with ADHD are excessively hyperactive or impulsive and inattentive. Compared to their peers, they show considerable difficulties in maintaining visual attention and are frequently involved in ‘off-task’ behaviour. One hypothesis states that ADHD reflects a core deficit in inhibitory control.

Being blind is not officially labelled as a disorder, although it can cause developmental differences. Children who are congenitally blind (blind from birth) often also have a different disorder, making it difficult to distinguish the effect of blindness on development. Many mechanisms of pre-linguistic communication are visually based. Eye contact is used to establish and maintain exchanges between the caretaker and the infant. Non-visual perceptual information is sufficient for establishing exchanges between blind infants and their parents. Person-person-object style interactions, interactions in which there is joint attention towards an object, are not established reliably until 21 months of age in blind infants. These interactions were mostly established using touch, letting the infant touch the object, rather than pointing at it and using vision.

Around 50 percent of children who are registered blind have other disabilities. For children with a dual-sensory impairment (e.g: being blind and deaf), it is important to adapt to the environment within an educational setting. This can facilitate the child’s independence in moving around their learning environment.

Some forms of atypical development have a clear genetic basis, while for others, environmental input is a major contributory factor. Bronfenbrenner’s ecological model is an influential model that places individual human development into the wider context of interaction with the immediate environment as well as the larger context. It consists of several systems:

1. Microsystem
   This is the individual child and their characteristics and the setting where interpersonal relationships relevant at different points in the lifespan take place.
2. Mesosystem
   This refers to the relation between different microsystems (e.g: relation between school and family).
3. Exosystem
   This refers to the variety of influences, such as mass media, the neighbourhood and family friends.
4. Macrosystem
   This refers to the cultural level of influence.
5. Chronosystem
   This refers to the sociocultural events over the life course

The systems are an increasingly less direct influence on the child’s development but are nevertheless relevant.

Children that lived in Romanian institutions for at the six months of life had a longer-term impact. They had an attachment disorder in the form of disinhibited attachment. The earlier a child was rescued from the institution, the less damage was done. Children that were rescued before the age of 6 months had a normal development, where other children had atypical developments.