Less trees - more ADHD

This has been demonstrated by a research project conducted by iPSYCH, the Lundbeck Foundation's Initiative for Integrative Psychiatric Research. However, the researchers are not – yet – able to give us a definite answer as to why this should be.

Building plots are expensive in big cities, which is why we build homes close together when we establish new residential areas.

This is true of cities all over the world, but it is at the expense of our cities’ green spaces. It often means that city-dwellers do not see many trees or other natural features when they step outside their door.

And it appears that this absence of nature in our daily lives has a negative impact on the incidence of ADHD. A new research project at Aarhus University (AU) shows that the less children experience nature in their local environment during the first five years of their lives, the greater the risk that they will later be diagnosed with ADHD:

Children who grow up in an environment with very little nature have a 20% higher risk of developing ADHD than children who live in a green area with lots of nature.

She is the first author of the study that was recently published in the scientific journal Environmental Health Perspectives.

The study involves health data from 815,000 Danes born between 1992 and 2007. This is the largest study yet of potential correlations between the absence of green nature in our daily lives and the risk of developing ADHD.

The large-scale analytical work was partially financed by research funding from iPSYCH, the Lundbeck Foundation Initiative for Integrative Psychiatric Research.

How do they calculate it?

The arithmetic behind the study required some sharpening of pencils. There were lots of different calculations to be compared.

The study was actually only possible because the civil registration number (CPR number) of every single Dane is attached to information about their health, finances, level of education and place of residence – from cradle to grave.

And because researchers have special permission to access – in anonymised format – these details as well as similar information about the individual’s parents.

The study, headed by Malene Thygesen, is a so-called population-based cohort study.

This means that the researchers follow a section of the population to investigate a specific issue – in this case, the development of health in relation to ADHD.

Malene Thygesen and her colleagues had access to the anonymised, CPR-based health data of the 815,000 Danes born between 1992 and 2007, and one of the questions they asked was:

‘How many of the 815,000 were diagnosed with ADHD after they turned five?’

The answer was 30,000, so approximately 3.5%. And two out of three were boys, which is typical of the ADHD gender balance.

The AU researchers also conducted a very thorough study of the 815,000 individuals, looking at the extent to which they had access to green areas from their birth up to the age of five.

This was possible thanks to satellites.

Satellites are constantly passing over Denmark, and images taken by them enable us to determine how green a specific address is – so, how much green there is in its surroundings.

The images come from six American satellites, which have been taking photos of the Earth, including the surface of Denmark, for almost 40 years. The AU researchers were able to retrieve these data from the American Landsat Archive.

By pairing the satellite data with the anonymised CPR data from the Danish civil register, Malene Thygesen and her colleagues were able to get the computer to calculate for each of the 815,000 individuals whether they had had a “green address”. In other words, whether they had lived close to nature during the first five years of their lives.

Or whether trees, bushes and grass had been scarce in the environment in which they grew up.

It turned out that the risk of developing ADHD was significantly higher in the areas where nature was sparse. Malene Thygesen explains that, after correction for factors such as the socio-economic conditions of the families, the number was, in fact, 20% higher than in typically green residential areas:

‘We can’t say what the precise reason is for this difference. But, of course, we have some theories. For example, access to green surroundings promotes good mental health in children – and this could be a protective factor, reducing the risk of developing ADHD. Green areas give children opportunities to play outdoors, enabling them to develop physically and socially, and green areas also tend to have less noise and lower levels of air pollution than areas lacking in nature.’

About ADHD

• ADHD – Attention Deficit Hyperactivity Disorder – is the umbrella term for a variety of problems relating to attention, activity and impulsive behaviour.
• These may manifest in the form of physical agitation, concentration problems, lack of initiative, restlessness and a tendency to talk a lot. (Source: The Great Danish Encyclopaedia)
• It is thought that it may be possible to attach a diagnosis of ADHD to 3–5% of children in a year group. A smaller proportion of children and young people, approximately 1–2%, will have severe ADHD. (Source: Danish Health Authority)
• The number of children diagnosed with ADHD has been on the rise in Denmark in recent years. According to the Danish Health Authority, ‘it is estimated that the incidence of ADHD at a national level tripled’ between 2006, when 7,186 individuals were diagnosed with the disorder, and 2016 when the number was 25,029. (Source: Danish Health Authority)
• It is assumed that part of the explanation for this is that more are tested for ADHD these days than 10–20 years ago.
• Both hereditary factors and conditions associated with the child’s environment may play a role in the development of ADHD. Therefore, if we want to calculate the impact of a single factor on development of ADHD – in the case of the Aarhus researchers, presence or absence of a green residential area in a child’s early years – we need to correct for a number of factors.
• Some of the factors Malene Thygesen and her colleagues corrected for were: Gender; birth year; parents’ socio-economic status; and average socio-economic status in the municipalities from which the 815,000 individuals came.