In the early 1980s, the concept of an 'autism spectrum' was introduced by Lorna Wing and Gillberg. Gillberg has done extensive research into autism throughout his academic career. In 2003, a French and Swedish research team at the Institut Pasteur and the psychiatric departments at Gothenburg University and University of Paris, led by Thomas Bourgeron, Marion Leboyer and Gillberg, discovered the first precisely identified genetic mutations in individuals with autism. The team identified mutations altering two genes on the X chromosome which seem to be implicated in the formation of synapses (communication spaces between neurons), in two families where several members are affected. Previous studies, such as the Paris Autism Research International Sib-Pair Study (PARIS), coordinated by Gillberg and Marion Leboyer, have more generally associated the X-chromosome regions with autism. The 2003 breakthrough indicated the location of the mutation to be on the NLGN4 gene and the NGLN3 gene. The mutation prevents a complete protein from forming and is inherited from the mother.

Since 2006, Gillberg is involved in a large cross-disciplinary project titled "Autism spectrum conditions: the Gothenburg collaborative studies", financed by the Swedish Research Council (Vetenskapsrådet), expected to run until the end of 2009. The project is a collaboration between scientists specialized in child and youth psychiatry, molecular biology and neuroscience and involves a genetic part with an international study team of French, British and U.S. researchers examining various aspects autism. Some of the results were published during 2007. The project also includes a genetic study on the Faroe Islands.

In 1989, Gillberg became instrumental in the publication of the first diagnostic criteria for Asperger syndrome. Gillberg's criteria most closely resemble the original description of Hans Asperger, and for this reason, some clinicians consider them the first choice in clinical practice. All of the following six criteria must be met for confirmation of diagnosis:

1. Severe impairment in reciprocal social interaction (at least two of the following)
1. inability to interact with peers
2. lack of desire to interact with peers
3. lack of appreciation of social cues
4. socially and emotionally inappropriate behavior
2. All-absorbing narrow interest (at least one of the following)
1. exclusion of other activities
2. repetitive adherence
3. more rote than meaning
3. Imposition of routines and interests (at least one of the following)
1. on self, in aspects of life
2. on others
4. Speech and language problems (at least three of the following)
1. delayed development
2. superficially perfect expressive language
3. formal, pedantic language
4. odd prosody, peculiar voice characteristics
5. impairment of comprehension including misinterpretations of literal/implied meanings
5. Non-verbal communication problems (at least one of the following)
1. limited use of gestures
2. clumsy/gauche body language
3. limited facial expression
4. inappropriate expression
5. peculiar, stiff gaze
6. Motor clumsiness: poor performance on neurodevelopmental examination

Gillberg's criteria differ from those given in the DSM-IV-TR. Some scholars have therefore criticized them for "making it difficult to compare with other studies." It has been argued that the failure of some research groups to replicate some of Gillberg's findings "may relate primarily to fundamental differences in diagnostic approach".

In the 1970s, Gillberg played a leading role in developing the concept Deficits in Attention, Motor control and Perception (DAMP), a concept primarily used in Scandinavia. The DAMP concept as used in more recent publications, refers to Attention-deficit hyperactivity disorder (ADHD) in combination with Developmental Coordination Disorder (DCD). According to Gillberg, it constitutes a "subgroup of the diagnostic category of ADHD, conceptually similar - but not clinically identical - to the WHO concept of HKD (hyperkinetic disorder)" and is diagnosed on the basis of "concomitant attention deficit/hyperactivity disorder and developmental coordination disorder in children who do not have severe learning disability or cerebral palsy".

Some scholars disagree with the lumping of ADHD and DCD, with the argument that they are unrelated. Gillberg stated in 2003 that, although he feels that there is a "very real issue of how to deal with the conflict between splitting (ADHD plus developmental coordination disorder (DCD)) and lumping (DAMP)," he nevertheless feels that "the DAMP construct has been helpful in identifying a group of children with ADHD and multiple needs that will not be self evident if the diagnosis is just ADHD or just DCD." Before the Scandinavian studies, recognition that individuals with attention problems may also have difficulties with movement, perception, and memory had received little attention in studies. According to various studies, half of the children with ADHD also have DCD.

With the development of the ADHD concept, the previous, less precise, category of Minimal Brain Dysfunction (MBD), "a term almost universally employed in child psychiatry and developmental paediatrics from the 1950s to the early 1980s" was replaced. Gillberg began to study DAMP in the late 1970s, when ADHD was still called MBD and the DAMP concept has been adjusted as the term ADHD was introduced and became internationally used. Around 1990, DAMP had become a generally accepted diagnostic concept in two Nordic countries, but when the DSM-IV appeared in 1994, DAMP became considered a redundant term in many countries, since DAMP is essentially equivalent to ADHD in combination with DCD as defined by DSM-IV. Gillberg's four criteria for DAMP are:

• ADHD as defined in DSM-IV;
• Developmental Coordination Disorder as defined in DSM-IV;
• Condition not better accounted for by cerebral palsy; and
• IQ higher than about 50 [Gillberg, 2003: box 1].

According to Gillberg, clinically severe form DAMP (or ADHD+DCD) affects about 1.5% of the general population of school age children; another few per cent are affected by more moderate variants. Boys are overrepresented; girls are currently probably underdiagnosed. There are many overlapping conditions, including conduct disorder, depression/anxiety, and academic failure. There is a strong link with autism spectrum disorders in severe DAMP. Familial factors and pre- and perinatal risk factors account for much of the variance. Psychosocial risk factors appear to increase the risk of marked psychiatric abnormality in DAMP. Outcome in early adult age was psychosocially poor in one study in almost 60% of unmedicated cases. About half of all cases with ADHD have DCD, and conversely, ADHD occurs in about half of all cases of DCD.

Gillberg has published around 80 papers on DAMP, ADHD and related conditions.

One of Gillberg's research projects, the Gothenburg study, has become the center of a heated controversy. The controversy concerns the question to what extent the Principle of Public Access, which in Sweden supports transparency in publicly funded activities, can be applied to sensitive data collected in medical studies involving human subjects. In 2003, Gothenburg University was ordered by the court to release medical records and other sensitive data about a group of children who had participated in a longitudinal psychiatric study done by Gillberg and other researchers, to two individuals under the Freedom of Information Act; this was done despite the researchers' assertion that anonymization was not considered feasible due to the nature and length of the study (a small group of participants had been followed for a period of 16 years and the data included a combination of taped interviews, medical records, criminal records, school records, and psychiatric evaluations). The court ordered the University to set conditions for the access so that the interests of the children and the families would be protected.

In April 2003, the University's Vice-Chancellor set the conditions: one of the persons requesting access, the sociologist Eva Kärfve, would have to get her research project approved by the ethical review committee, and each concerned individual would have to consent before documents about her or him could be read by Eva Kärfve and Leif Elinder, the other person who had requested access. However, Kärfve and Elinder appealed the University's conditions and the Administrative Court of Appeal ruled that the conditions were unreasonable. In an analysis of the case, Sven Ove Hansson, Professor and Head of the Department of Philosophy and the History of Technology at the Royal Institute of Technology (KTH), Stockholm, a former member of the Swedish Government Research Advisory Board, wrote: "[I]t is particularly interesting to note that the Court of Administrative Appeal nullified the decision by Gothenburg University to require individual consent and approval from an ethical review committee before giving access to sensitive data on individual research subjects. These are two of the cornerstones of the scientific community's own system for protecting research subjects."

Background and legal battle

Beginning in 1996, pediatrician Leif Elinder criticized Gillberg's research and alleged that the numbers reported by Gillberg were made up.