The present register-based study investigated the role of IQ in predicting a wide range of indicators of unsuccessful educational and occupational achievement among young men born across five decades in Denmark. The study population comprised all Danish men born since 1950 and appearing before a draft board during the periods from 1968-1984 and from 1987-2015 (N=1,098,742). IQ was measured by Børge Priens Prøve at age 18. Unsuccessful educational achievement was indicated by leaving lower secondary school without a certificate, by no completed youth education at age 25, by no completed education leading to vocational qualifications at age 30, and by number of interruptions to education at age 30. Unsuccessful occupational achievement was indicated by being not in employment, education or training (NEET) at age 30, by unemployment at age 30, by receiving sickness benefits at age 30, by receiving welfare benefits at age 30, by receiving disability pension at age 30, and by gross income at age 30. Binary logistic regression, negative binomial regression, and median regression were used to estimate the associations of IQ with unsuccessful educational and occupational achievement. The results showed that low IQ was a strong predictor of all indicators of unsuccessful educational and occupational achievement. With regard to unsuccessful educational achievement, the probabilities of no school certificate, no youth education, and no vocational qualification decreased with increasing IQ in a cubic relation, suggesting essentially no or only weak associations at superior IQ levels. IQ had the strongest influence on the probability of no school certificate. Although the probabilities of the three outcomes were almost the same among individuals with extremely low IQ, the probability of no school certificate approached zero among individuals with an IQ of 100 or above whereas the probabilities of no youth education and no vocational qualification remained notably higher. The number of interruptions to education at age 30 decreased linearly with increasing IQ. Thus, individuals with an IQ of 70 had 0.72 interruptions, individuals with an IQ of 100 had 0.65 interruptions, and individuals with an IQ of 130 had 0.58 interruptions to education. With regard to unsuccessful occupational achievement, the probabilities of NEET, unemployment, sickness benefits, and welfare benefits decreased with increasing IQ in a cubic relation and the probability of disability pension decreased with increasing IQ in a quadratic relation, suggesting much weaker associations than for the indicators of unsuccessful educational achievement. Gross income increased with increasing IQ in a statistically significantly cubic relation. Thus, individuals with an IQ of 70 had a median gross income of 301,347 DKK, individuals with an IQ of 100 had a median gross income of 331,854 DKK, and individuals with an IQ of 130 had a median gross income of 363,089 DKK – corresponding to about 47,856 USD, 52,701 USD, and 57,662 USD, respectively. Overall, it seemed that IQ had the strongest influence on the risk of unsuccessful educational achievement and on the risk of disability pension. The influence of IQ on educational achievement was strongest in the early educational career and decreased over time. No consistent time trends in the investigated associations were found. In conclusion, the study findings suggest that assessment of intelligence may provide crucial information for the counselling of poor-functioning schoolchildren and adolescents with regard to both the immediate educational goals and the more distant work-related future.
