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The Christchurch Health and Development Study (CHDS) is a longitudinal study that investigated the development and mental health of 1265 children, born and raised in Christchurch, New Zealand. The children were studied over a span of 40 years (in 2019). The aim was to establish causality between several disorders, such as anxiety disorders, mood disorders, conduct disorder, substance-related disorders, and contributing factors. It began in 1977 and was first led by David M. Fergusson and later, taken over by Professor Joseph Boden.

Context
The Christchurch Health and Development Study arose out of a study from Fred Shannon, who was the Foundation Professor of Paediatrics at Christchurch Hospital. It caught his attention that the majority of his patients that suffered from problems regarding health and development came from single-parent families rather than from families with two parents. Together with the Medical Research Council, this concept was transferred into a longitudinal study design.

In 1976, David M. Fergusson joined the Christchurch Child Development Study which later became the Christchurch Health and Development Study. Fergusson worked as an Emeritus Professor at the University of Otago and led the CHDS until his retirement in August 2015. Afterwards, Professor Joseph Boden took over his position as the director of the Christchurch Health and Development Study.

Method
The CHDS is a longitudinal cohort study. The initial cohort included children that were born in Christchurch, New Zealand from april 15th to august 5th 1977. From the 1310 children born during that period, 97% (1265 children) were included in the cohort. At age 35, 962 (79%) of participants remained.

Assessments were made at birth, 4 months and then at annual intervals until the age of 16. The following interviews were conducted at the ages of 18, 21, 25, 30 and 35. In 2019 the interviews for the 40 year assessment were completed.

Data was collected through parental interviews, teacher questionnaires, child interviews, hospital records and police record data. In 2020 more than 50 million characters of information have been gathered and over 500 scientific papers and books have been published.

Results
The Christchurch Health and Development study conducted a variety of research with a diverse pool of topics over the last 40 years. Some major results are listed below.

One of their research objectives were studies about child and adolescent mental health. What they found was that at the age of 15 around one quarter showed signs of psychiatric disorders according to DSM-III-R and DSM-IV criteria. Those disorders included anxiety disorders, mood disorders, conduct disorder and substance-related disorders etc.. Females generally showed a higher rate of psychiatric disorders, especially mood disorders and depression.

When surveyed again at the age of 18, the studies showed an increase of cases with a disorder to around 40% of the total cohort. Especially substance abuse and internalizing disorders were more profound. However, at this age there wasn’t a great difference between males and females to be found anymore. Out of those 40% with a psychiatric disorder, less than one quarter sought treatment, either because they didn’t think it necessary or because they thought the problem would resolve itself. The ones that did seek treatment were usually the ones that suffered from mood disorders that were highly impairing their lives and the ones that had a previous history of psychiatric contact.

Furthermore they investigated the consequences of mental health problems and the development of suicidal behaviours. Up until the age of 21 over ¼ of the participants reported suicidal thoughts and nearly 8% even attempted suicide. The main reasons for this were psychiatric disorders but also personality factors and the exposure to adverse childhood circumstances and life events.

The longitudinal study design also allowed for research concerning antecedent risk factors and their outcomes. The examination of the teeth of the children of the cohort showed that low levels of lead exposure correlated with small deficits in school achievements and IQ that were still evident at the age of 18. There was also an increased rate of behaviour disorders evident in those individuals.

Another research topic was the effect of physical and sexual child abuse on mental health. The results showed that especially sexual abuse lead to an increased risk of many psychiatric disorders such as depression, anxiety, conduct disorder, as well as substance abuse and suicidal behaviour. Sexual abuse also proved to be among the most detrimental adverse childhood experiences with its effects evident up to 20 years or even more. Physical abuse did also lead to an increased risk of mental disorders, however, those could be mainly explained by other social and contextual factors (e.g. family dysfunction, impaired parenting, parent psychopathology).

In general those individuals with the highest exposure to adverse childhood experiences were 100 times more at risk for problem behaviours later in life, compared to the more advantaged half of the cohort.

Apart from the mental health of its' participants, the CHDS also conducted research about the relation between parental smoking and lower respiratory diseases in children. They found that children of smokers were at increased risk of bronchitis and pneumonia and that children of women who smoked during pregnancy were more prone to develop psychiatric disorders especially conduct disorder later in life. In the long run individuals with conduct problems also appeared to be more susceptible to criminal offending, delinquency, substance abuse and the externalization of problems , while early attentional problems were a precursor of later educational difficulties.

Reception
The use of a longitudinal design for the CHDS yielded various methodological advantages. It enabled the analysis of many factors on mental health and development, so that different disorders and their prevention could be examined. The design allowed the researchers to introduce causality of these disorders and the determination of their precise onset and offset times. It facilitated the investigation of early childhood experiences and their consequences in adolescence, including family environment, child abuse and substance abuse in later years. Compared to other longitudinal studies, the CHDS has a relatively small sample which prevented it from studying disorders that have a lower prevalence, such as autism.

The CHDS shows how findings from longitudinal studies can be used for the development of effective evidence-based prevention and treatment programmes.

Apart from providing scientifically relevant findings, it also created awareness for the importance of mental health and how influencing factors can lead to severe consequences later in life. Furthermore, it contributed to important improvements of social and health policies as well as the creation of guidelines. The Early Start Project, a service for high-needs families, was founded based on the CHDS findings. It became one of the most efficient home visitation programs worldwide, serving as an example for the study’s achievements. Findings about how disadvantaged families make use of childhood services, how conduct problems in younger years affect later development, the importance of early intervention to avoid multiple problems or long term effects of cannabis use have contributed to the development of policies in these areas. Other findings have influenced many public health issues, for example swimming pool safety, effects of passive smoking on children and more.

Awards
New Zealand Medical Journal in 2019