Age at school entry and reported symptoms of attention-deficit/hyperactivity in first graders: results of the prospective cohort study ikidS

Systematic review and meta-analysis: relative age in attention-deficit/ hyperactivity disorder and autism spectrum disorder

Youngest students in their class, with birthdates just before the school entry cut-off date, are overrepresented among children receiving an Attention-Deficit/Hyperactivity Disorder (ADHD) diagnosis or medication for this. This is known as the relative age effect. This systematic review and meta-analysis summarises the evidence on the influence of relative age on ADHD symptoms, diagnosis and medication prescribing. As no review to date has investigated the association with autism spectrum disorder (ASD) diagnosis, this is also examined. Following prospective registration with PROSPERO, we conducted a systematic review according to the PRISMA guidelines. We searched seven databases: Medline, Embase, PsycInfo, Web of Science Core Collection, ERIC, Psychology and Behavioural Sciences Collection, and Cochrane Library. Additional references were identified from manual search of retrieved reviews. We performed a meta-analysis of quantitative data. Thirty-two studies were included, thirty-one investigated ADHD and two ASD. Younger relative age was associated with ADHD diagnosis and medication, with relative risks of 1.38 (1.36-1.52 95% CI) and 1.28 (1.21-1.36 95% CI) respectively. However, risk estimates exhibited high heterogeneity. A relative age effect was observed for teacher ratings of ADHD symptoms but not for parent ratings. With regard to ASD, the youngest children in their school year were more likely to be diagnosed with ASD. This review confirms a relative age effect for ADHD diagnosis and prescribed ADHD medication and suggests that differences in teacher and parent ratings might contribute to this. Further research is needed on the possible association with ASD.

Relative Age Effect in Attention Deficit/Hyperactivity Disorder at Various Stages of the Medicalization Process

Diagnosis and pharmacological treatment of ADHD are more common among the youngest children in a classroom, born in the months immediately preceding the school entry cutoff date. The mechanisms behind this phenomenon, called the relative age effect (RAE), are not yet well understood. Nearly all hypotheses involve the school system, various teachers’ actions, and concern children’s immaturity. However, most previous studies have been based on reports of health professionals’ diagnoses and prescriptions found in official databases rather than on reports of teachers’ behavior identification or suspicion of ADHD, despite their being at the first stages of the medicalization process. Our study overcomes this limitation by using reports of parents’ and teachers’ behavior identification or suspicion of ADHD within a three-level multilevel survey design, comprising 1294 children, 130 teachers, and 17 elementary public schools. The goal of our study was to investigate whether RAE stems (1) from adults’ judgement of the child’s expression of immaturity or (2) from the consequences associated with the child’s double burden of being immature and exhibiting ADHD behaviors. Our multilevel analyses put forward the first hypothesis only, supporting the medicalization of immaturity. RAE in ADHD seems mostly initiated by teachers’ pre-diagnostic actions toward younger children.

The impact and causal directions for the associations between diagnosis of ADHD, socioeconomic status, and intelligence by use of a bi-directional two-sample Mendelian randomization design

BACKGROUND

Previous studies have reported associations between attention-deficit/hyperactivity disorder (ADHD) and lower socioeconomic status and intelligence. We aimed to evaluate the causal directions and strengths for these associations by use of a bi-directional two-sample Mendelian randomization (MR) design.

METHODS

We used summary-level data from the largest available genome-wide association studies (GWAS) to identify genetic instruments for ADHD, intelligence, and markers of socioeconomic status including the Townsend deprivation index, household income, and educational attainment. Effect estimates from individual genetic variants were combined using inverse-variance weighted regression.

RESULTS

A genetically predicted one standard deviation (SD) increment in the Townsend deprivation index conferred an odds ratio (OR) of 5.29 (95% confidence interval (CI) 1.89-14.76) for an ADHD diagnosis (p<0.001). A genetically predicted one SD higher education level conferred an OR of 0.30 (95% CI 0.25-0.37) (p<0.001), and a genetically predicted one SD higher family income provided an OR of 0.35 (95% CI 0.25-0.49; p<0.001). The associations remained after adjustment for intelligence whereas the lower odds of an ADHD diagnosis with higher intelligence did not persist after adjustment for liability to greater educational attainment (adjusted OR 1.03, 95% CI 0.68-1.56; p=0.87). The MR analysis of the effect of ADHD on socioeconomic markers found that genetic liability to ADHD was statistically associated with each of them (p<0.001) but not intelligence. However, the average change in the socioeconomic markers per doubling of the prevalence of ADHD corresponded only to 0.05-0.06 SD changes.

CONCLUSIONS

Our results indicate that an ADHD diagnosis may be a direct and strong intelligence-independent consequence of socioeconomic related factors, whereas ADHD appears to lead only to modestly lowered socioeconomic status. Low intelligence seems not to be a major independent cause or consequence of ADHD.