Childhood adversity is linked to cognitive ability
Young people who grow up with multiple and long-term family-related adversities score lower on intelligence tests. This is shown by a new register-based study of more than 360,000 young Danish men whose IQ was measured at the Danish military conscription assessment.
Severe and prolonged adversity in the family during childhood – such as poverty, illness, or unstable and insecure living conditions – can leave lasting marks. A new Danish study shows that young people who have experienced such adversities have, on average, a lower IQ than peers who grew up without significant hardship.
The study, which is the largest of its kind, was conducted by the Danish National Institute of Public Health and has just been published in the scientific journal Pediatrics Open Science.
PhD student Rebecca Beatrix Clarke, one of the researchers of the study, acknowledges that comparing people’s intelligence can be a sensitive issue. Nevertheless, there are good reasons to examine the link between childhood conditions and cognitive development.
“Cognitive ability, often measured with IQ tests, are associated with many important outcomes later in life, such as educational attainment, employment, quality of life, health, and life expectancy. This makes it important knowledge if we want to prevent inequality and give children the best possible opportunities,” she says.
Examining childhood adversity among young men
The study includes 368,165 young men born in Denmark who participated in the Danish military conscription assessment between 2006 and 2022, where IQ testing is a standard part of the evaluation.
Data from more than 20,000 young women who voluntarily participated in the assessment are also included in the study.
Using Danish national registers, the researchers mapped the participants’ childhood up to the age of 15 to identify patterns of childhood adversity, such as poverty, serious illness in the family, parental mental illness and substance abuse, and out-of-home placement.
These data were then linked to their IQ scores measured in young adulthood at the conscription assessment.
According to Rebecca Beatrix Clarke, the results show a clear association between family-related adversity during childhood and lower IQ in young adulthood.
“We find that young people without significant childhood adversity have an average IQ of 102. In comparison, those with the highest and most prolonged levels of adversity score on average 6.4 IQ points lower, even after adjusting for a range of family-related factors,” she explains.
Children who experience multiple forms of adversity over many years appear to be particularly affected.
“Among children who experienced less severe adversity over shorter periods, we also see lower intelligence, but not to the same extent as among those exposed to the highest levels of adversity,” she says.
According to Rebecca Beatrix Clarke, the differences in IQ for individuals may not be large, but even small differences in average IQ can have significant implications at the population level.
“Intelligence matters not only for individuals, but also for society as a whole. Socioeconomic studies show, for example, that even small increases in the average IQ of a population are associated with higher economic and societal growth.”
Stress affects the brain
According to Rebecca Beatrix Clarke, there are several possible explanations.
“We know that stress affects the brain, especially in children. If a child is exposed to severe stress over many years due to poverty, illness, or an unstable family environment, the body is in a constant state of alert, which can affect brain development and cognitiveability,” she says.
In addition, growing up in unstable and insecure environments may be associated with fewer resources and less capacity among caregivers to support, read to, talk with, and stimulate the child. Parental education may also play a role:
“Even after adjusting for parental education and a range of other family-related factors, we still see a consitent association. This suggests that the childhood environment itself plays an important role,” Rebecca Beatrix Clarke explains.
The importance of prevention
According to the researchers, the study adds to a growing body of evidence showing that childhood is crucial for later life outcomes, and that early interventions are important.
“Our results can be used as a basis for supporting children and families in difficult circumstances at an early stage. This also includes prioritising high-quality daycare and schools to ensure that all children have the best possible opportunities,” says Rebecca Beatrix Clarke.
Facts
How IQ is measured at the Danish conscription assessment:
IQ is measured using the Børge Prien’s Test. The test is time-limited and consists of tasks involving words, numbers, figures, and patterns. Participants receive a total score, which was converted into a standard IQ scale with an average of 100, where most people score between approximately 85 and 115.
Contact: PhD student Rebecca Beatrix Clarke, tel.: +45 6550 1682, e-mail: rbc@sdu.dk, Senior researcher Gunhild Tidemann Okholm, tel.: +45 6550 7770, e-mail: gund@sdu.dk, Professor Trine Flensborg-Madsen, tel.: +45 6550 9361, e-mail: tfl@sdu.dk. Danish National Institute of Public Health, SDU