The Norwegian Armed Forces Health Registry and use of military screening data to evaluate changes in IQ scores over time

Abstract

Introduction Up to the year 2009, all young Norwegian men were obliged to attend military medical examinations to assess their suitability for military service. Since 2010, both sexes have been required to complete a health-assessment questionnaire for the Norwegian conscription board when they turn eighteen. Based on the responses, a portion is called for comprehensive medical examinations. These examinations include tests of physical fitness and intellectual performance, review of any documented disease or disability, general clinical examinations and interviews, tests of visual acuity and colour vision, and audiometry. The data are stored in the Norwegian Armed Forces Health Registry (NAFHR) in accordance with the Act on Health Registers and Health Information. The register is maintained for the purpose of monitoring the health of the Armed Forces Personnel and identifying risk factors associated with military service. Currently, the NAFHR contains health data on 1.2 million men and 63,500 women born between 1926 and 1998. The annual coverage varies as military screening is constantly changing in line with the Armed Forces’ need for personnel. The NAFHR includes data on 90–95% of the annual male birth cohorts born between 1950 and 1991. The data collected at the military medical examinations has been applied in public health research since 1996. This is made possible by linking the NAFHR data with national registries on health, education, work and social benefits. The research has provided insight into i.e. how factors related to neonatal life/birth and family affects the adult body and cognitive performance and has helped to understand how mental and physical health in adolescence is associated with important public health problems in adulthood. Similar data are available in several European countries, enabling large multinational studies and comparisons between countries. In light of the recent debate on possibly decreasing trends in intelligence scores in the Scandinavian countries, we examined secular trends of intellectual performance in Norway. Methods The military's tests for intellectual performance include problems related to number-series (calculation), verbal analogues and geometric figures. A combined score for the three tests is given as a single-digit standard nine (“stanine”) scale ranging from 1 (lowest) to 9. The scores are normally distributed with a mean of 5.0 and SD = 2.00. A stanine score of 5 corresponds to 100 points on the intelligence quotient (IQ) scale (WAIS), with an increment/decrement of 7.5 IQ points for each stanine unit deviating from 5. Our study included 493,615 men tested for intellectual performance at a conscription board between 1980 and 2009 at the age of 18. We divided the study period into six five-year periods and used one-way between-group analysis (Welsh) with Tukey post hoc tests to examine differences in mean stanine score of intellectual performance across the periods. P-value Results Mean stanine scores of intellectual performances were statistically significant different between the periods. The score increased from 4.93 (SD = 1.8) in 1980–84 to 5.12 (SD = 1.8) in 1985–89 and peaked at 5.29 (SD = 1.9) in 1990–94. Thereafter, the mean stanine score fell to 5.13 (SD = 1.8) in the period 2000–04 and to 4.97 (SD = 1.7) in 2005–09. When referenced to the IQ scale, our results show that men tested in 1990–94 scored on average 2.7 points higher than those tested in 1980–84. Men tested in 2005–09 scored on average 2.4 IQ-points lower than those of the peak period (1990–94), but still 0.3 IQ-points higher than those who were tested in the first period. Conclusion We observed a significant gain in intellectual performance among 18-year-old men between 1980 and 1994, thereafter a similar and significant loss until 2009.