QJM Advance Access originally published online on May 25, 2007
QJM 2007 100(7):405-414; doi:10.1093/qjmed/hcm039
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Childhood mental ability in relation to cause-specific accidents in adulthood: the 1970 British Cohort Study
From the 1MRC Social & Public Health Sciences Unit, University of Glasgow, Glasgow,2Department of Psychology, University of Edinburgh, Edinburgh,3Department of Psychology, City University, London, and 4MRC Epidemiology Resource Centre, University of Southampton, Southampton General Hospital, Southampton, UK
Address correspondence to Dr G.D. Batty, MRC Social & Public Health Sciences Unit, University of Glasgow, 4 Lilybank Gardens, Glasgow G12 8RZ. email: david-b{at}msoc.mrc.gla.ac.uk
Received 15 December 2006 and in revised form 15 February 2007
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Summary |
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Background: Few data link childhood mental ability (IQ) with risk of accidents, and most published studies have methodological limitations.
Aim: To examine the relationship between scores from a battery of mental ability tests taken in childhood, and self-reported accidents between the ages of 16 and 30 years.
Methods: In the British Cohort study, a sample of 8172 cohort members born in Great Britain in 1970 had complete data for IQ score assessed at 10 years of age and accident data self-reported at age 30 years.
Results: The relationship between childhood IQ score and later risk of accident was complex, differing according to sex and the type of accident under consideration. Women with higher childhood IQ were more likely than those with lower scores to report having had an accident(s) while at work, in a vehicle, engaging in sports, and in unspecified circumstances. Adjustment for markers of socioeconomic position weakened or eliminated some of these relations, but higher childhood IQ remained associated with increased risk of sporting and unspecified accidents. Men with higher childhood IQ scores were less likely than those with lower scores to report accidents at work, but more likely to report accidents at home, playing sports or in unspecified circumstances. After adjustment for socioeconomic circumstances, higher childhood IQ in men remained associated with an increased risk of accidents at home or in unspecified circumstances.
Discussion: The relationship between childhood mental ability and accidents in adulthood is complex. As in other studies, socioeconomic position has an inconsistent relationship with non-fatal accident type.
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Introduction |
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In Western societies, unintentional injury (accidents), closely followed by homicide and suicide, is the leading cause of death in children and young people.1–3 As a consequence, accidents, together with cardiovascular disease, cancer and mental illness, have been highlighted by the UK government as a key target for prevention.4 Despite their obvious public health significance, much less is known about the aetiology of accidents than is the case for these chronic diseases. In the few studies conducted, risk factors for fatal accidents combined, and those ascribed to specific causes (such as motor vehicle accidents), appear to include young age, male gender, ethnicity, living alone and socioeconomic disadvantage.5,6 When non-fatal accidents are the outcomes of interest, the link with these risk indices appears to be more complex.7 For instance, while socioeconomic advantage seems to be associated with protection from violent accidents, it is related to an increased risk of sporting injuries.8,9
In these studies, socioeconomic disadvantage has typically been indexed by manual occupation, low income, and basic educational credentials. Given its well-established correlation with these indices,10 particularly educational achievement,11 a relation between mental ability (as measured using IQ-type tests) and accident risk that followed the same direction as that for socioeconomic position might reasonably be hypothesized. There are at least two further reasons to anticipate a link between mental ability and accidents. Firstly, poor perceptual abilities, such as below average information processing speed, have been advanced as markers of risk in motor vehicle accidents, the major source of accidents in young persons.12 Processing speed and IQ correlate, such that persons with higher scores on standard mental ability tests have greater speed.13 Secondly, in persons with intellectual disability who occupy the extremely low end of the mental ability continuum, accidents rates are markedly elevated in comparison to the general population or normal controls.14,15
To date, most investigators examining the association between low mental ability and accidents have focused on motor vehicle accidents16,17 and related driving offences18,19 as the outcomes of interest. An inverse association is generally reported, such that persons with high mental ability scores experience fewer accidents and convictions. While motor vehicle accidents are a major contributor to the occurrence of all accidents in young adults, other important causes, which have as yet been unexplored in relation to mental ability, include accidents occurring within the workplace, the home and while at leisure. Furthermore, existing studies are subject to some methodological limitations. Firstly, several draw on select groups (e.g. army recruits,16 university students20) in which the range of mental ability scores will be narrower than in population-based cohorts, probably resulting in an underestimation of the strength of any IQ-accident association, should it be present. Secondly, most studies include only men;16–19 the relationship between low mental ability and accidents in women is largely unknown. Thirdly, the role of socio-economic position— a candidate confounder given its relation to both mental ability and accidents—is sometimes not taken into account in studies of mental ability and accident risk. Fourthly, most studies use a cross-sectional design. Given that head trauma (which is common, for instance, in motor vehicle accidents) often leads to reduced cognitive function,12 a spurious negative mental ability-accident gradient may be generated.
In the present study, we address these study limitations and paucity of data by relating mental ability at 10 years of age to a range of self-reported causes of non-fatal, unintentional accidents in young adult male and female participants (age 30 years) drawn from a nationally representative sample of the population of Great Britain who are well characterized for socio-economic circumstances across the life course. To our knowledge, this is the largest study to date to examine the relationship between pre-adult mental ability and accidents.
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Methods |
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The 1970 British Cohort Study is an on-going longitudinal study which takes as its subjects 17 198 live births occurring to parents residing in Great Britain between 5th and 11th April 1970.21 Following the initial 1970 survey, there have been five major follow-up studies of the cohort members to monitor their physical, educational and social development, their transitions to adult life22 and, in the latest study, their adult identity. These studies were conducted in 1975 (when the cohort members were aged 5 years), 1980–1 (aged 10), 1986 (aged 16), 1996 (aged 26) and 1999–2000 (aged 30). For the surveys in 1975, 1980–1 and 1986, the cohort was augmented by the inclusion of immigrants to Britain who had also been born in the target week in 1970. The present analyses use data from 1980–1 when study participants completed cognitive ability tests at age 10 years,23 and from 1999–2000 when, aged 30 years, they responded to enquiries about accidents.24
Data collected at the 10-year follow-up
Written informed consent was given by parents of study participants prior to the start of the study. Testing of the children's mental ability took place in schools. Mental ability at the age of 10 years was assessed using a modified version of the British Ability Scales (BAS),25 the administration of which was adapted so that it could be carried out by teachers. There were four sub-scales: word definitions, word similarities, recall of digits, and matrices. The word definitions sub-scale consisted of a list of 37 words. The teacher articulated each word in turn and asked the child about its meaning. The word similarities sub-scale consisted of 42 items each made up of three words (for example, orange, banana, strawberry, or sad, worried, happy). For each item, the teacher enunciated the three words and asked the child to name another word consistent with the theme. The recall of digits sub-scale consisted of 34 items. For each item, the teacher read out digits at half-second intervals and asked the child to repeat them. The matrices sub-scale consisted of 28 incomplete patterns arrayed as a grid. For each item, the child was requested to draw in the missing part of the pattern. Test results were scored by trained coders. Reliability of coding was monitored throughout the survey and results fed back to the coders. Regular checks carried out on a 5% random sample of tests showed that the percentage of tests where the original code was not confirmed on recoding was low: 0.8% for recall of digits, 4.7% for word definitions, 1.9% for word similarities and 2.3% for matrices.26
We used a principal components analysis of these four tests to establish the presence of a general cognitive ability factor. Examination of the scree slope suggested the presence of a single component. The first unrotated principal component accounted for 57% of the total variance among the four tests. The factor loading of each of the tests on the first unrotated principal component was 0.74 for matrices, 0.58 for digit recall, 0.83 for word definitions and 0.84 for word similarities. Scores were saved for each subject on the first unrotated principal component, which is an indicator of each person's general cognitive ability (g). For ease of interpretation, we transformed this index into an IQ equivalent score, giving it a mean of 100 and a standard deviation of 15. Each of the four mental tests is composed of two types of reliable variance: that which is shared with the other tests (g) and mental ability variance specific to each individual test. In order to explore whether any specific (non-g) aspect of mental performance on the cognitive tests had an influence on the study outcomes, we computed scores for the specific ability aspect of each of the four tests. We did so using linear regression, with the individual mental test as the dependent variable and g as the independent variable. From these regression analyses, we saved the residual scores, which represent how well each person performed on non-g aspects of the test.
Information on current occupation of both parents was collected during the interview with the child's parents. Social class of each parent was derived from current occupation, and based on four categories according to the 1980 Registrar General's Classification of Occupations.27 We used father's social class to define parental social class, or mother's social class where no father was present.
Data collected at the 30-year follow-up
Information was collected by interview in the participant's home. Participants gave written informed consent. As part of a series of questions on health, enquiries were made about accidents. Participants were asked if they had an accident for which they needed to see a doctor since the age of 16 years. If they responded positively, further enquiries were made as to the type of accident(s) based on the following pre-assigned categories: road accident as a pedestrian, road accident as driver or passenger, accident at home, accident at work, accident playing sport, or ‘other’. Participants were also asked about their highest academic or vocational qualification and these were coded into five categories (no qualifications; CSE grades 2–5/NVQ level 1 and equivalent qualifications; O levels/NVQ level 2 and equivalent qualifications; A levels/NVQ level 3 and equivalent qualifications; Degree or Diploma/NVQ level 4 and equivalent qualifications; Higher Degree, NVQ level 5). Social class was again derived from current occupation using four categories (professional/managerial, skilled non-manual, skilled manual, semi-skilled or unskilled). Participants were asked their current gross and net (take-home) pay from employment, and what period this amount covered. Annual equivalent last gross pay was derived from these data. When gross pay was missing but net pay data were available, gross pay was imputed from net using known parameters of the tax system in the relevant year.24
The number of participants in 1970 British Cohort Study at each wave of data collection is illustrated in Figure 1. Following an at-birth recruitment of 17 198 people, 14 875 children took part in the 10-year follow-up, representing 93% of those eligible to participate. Cognitive testing was completed on 11 563 (78%). By the time of the 30-year follow-up, the cohort consisted of 16 695 individuals. Of these, 14 087 cohort members could be traced, 13 394 were eligible to take part and 11 261 were interviewed for the 30-year follow-up, representing 84% of those eligible to participate. In all, 11 208 participants provided data on accidents, of which 8172 (73%) had data on IQ score at the age of 10 years and were therefore included in our analyses.
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Compared to these 8172 men and women, cohort members who did not participate in the 30-year follow-up had a slightly lower mean IQ score at age 10: mean (SD) IQ score was 97.1 (15.3) in non-participants compared with 101.1 (14.7) in participants (p value for difference <0.001). Those who did not take part in the 30-year follow-up were also slightly more likely to have had a manual social class background at the age of 10 than those who did participate (56.3% vs. 52.3%, p < 0.001). Some cohort members were missing data on parental social class at age 10 or current social class at age 30 years. Comparison of cohort members with and without complete data for these variables showed the relations between childhood IQ score and adult accident outcomes were similar in the two groups. In an effort to avoid potential selection bias, we retained cohort members with incomplete data in the analysis by creating an extra category for missing data within parental and current social class.
Statistical analyses
We used ANOVA and the 
2 test to examine the relation between IQ score and characteristics of the participants. Spearman correlation coefficients (rs) were used to examine the relations between IQ score and academic/vocational qualifications and income. The 2 test was used to examine the distribution of different accident types according to current social class. We used logistic regression to examine the relation of IQ score at age 10 years with the risk of different types of accident between the ages of 16 and 30 years. Risk estimates (odds ratios) are expressed per SD advantage (increase) in IQ score and are shown unadjusted and adjusted individually for social class (childhood and current), academic/vocational qualifications, annual earnings and, finally, for all these factors. Relative to the measurement of IQ at 10 years of age, in these analyses we conceptualized parental social class (ascertained at the same time as the IQ assessment) as a confounding variable, and current social class, educational attainment and income (assessed some years after IQ assessment) as mediating factors. With preliminary analyses revealing differences in the IQ-accident relations according to sex, results are presented for men and women separately. As described above, general cognitive ability, expressed here as IQ score, was derived from a principal components analysis of scores on the four tests making up the British Ability Scales. To explore whether performance on any of these tests had a specific influence on risk of accidents once the influence of general cognitive ability was subtracted, we saved the residuals from separate linear regression analyses of each test on IQ score. We used logistic regression to examine the relationship between these residuals and different types of accident.
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Results |
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Table 1 shows the characteristics of the study participants in relation to their IQ scores at age 10 years. IQ score at age 10 years was strongly associated with socioeconomic position across the life course. Thus, childhood IQ scores tended to be lowest in persons whose parents were from a manual occupational background, or who were currently in semi-skilled or unskilled jobs themselves. As expected, IQ scores were also positively correlated with the level of educational qualifications achieved. On average, males scored slightly higher on the IQ test than females. Data on current annual gross earnings were available for 5996 (74%) of the participants. This indicator of socioeconomic position was also positively related to childhood IQ test score (Spearman correlation coefficient 0.31, p < 0.001). The associations between IQ score and all markers of socioeconomic status were similar in men and women.
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Some 54.8% of the 8172 participants (69.7% of men and 40.8% of women) reported that they had had one or more accident since the age of 16 years for which they needed to consult a doctor. All types of accident were more commonly reported by men (p < 0.001 for difference for all). In Table 2, we show the number of accidents in total, plus the distribution of specific accidents in men and women according to socioeconomic position. For brevity, we restrict the results to the explanatory variable of current social class; similar patterns of association were evident for social class of origin, and adult education and income. Accidents at work were the most common type of accident in men (29%), followed by those while participating in sport (28.7%) and involving a vehicle (22.9%). In women, vehicle accidents were the most common type (17.7%), followed by accidents occurring at home (9.5%). Reports of accidents varied according to socioeconomic circumstances. Accidents while at work tended to be more common in men and women in manual or unskilled occupations. Sporting accidents by contrast were more common in men and women in professional/managerial occupations. Vehicle accidents showed a more complex pattern: in men, such accidents were most prevalent in study participants in skilled manual occupations, but in women, they were reported most frequently by those in professional/managerial occupations.
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In Table 3, we report the relation between IQ score at 10 years of age and self-reported accidents requiring medical attention since the age of 16 in men. There were no statistically significant associations between IQ score at age 10 years and the risk of having a road accident as a pedestrian or while in a vehicle. However, the risk of having an accident at work fell with increasing IQ score in an unadjusted analysis (OR 0.81, 95%CI 0.75–0.96). This effect was weakened following adjustment for parental social class or own educational attainment, and became non-significant after adjustment for current social class. By contrast, the prevalence of accidents while participating in sports (OR 1.25, 95%CI 1.16–1.34), while around the home (OR 1.12, 95%CI 1.01–1.24), or in circumstances other than those already specified (OR 1.15, 95%CI 1.01–1.31) was elevated in men who had higher ability at age 10 years. The relation between risk of sporting accidents and childhood IQ was attenuated when adjusted for all indicators of socioeconomic circumstances, but remained of borderline statistical significance. Further adjustment for the reported frequency of exercise participation at the time of the 30-year follow-up had little effect on this association. The increased risk of having accidents at home or in other, unspecified circumstances among men with higher childhood IQ was strengthened after adjustment. In multivariate analysis, for a one SD increase in IQ score at age 10, the risk of accident at home rose by 19% (OR 1.19, 95%CI 1.04–1.35), and risk of accident in other, unspecified circumstances by 29% (OR 1.29, 95%CI 1.08–1.53), after control for all indicators of socioeconomic conditions. In general, for those accident outcomes that revealed statistically significant associations with IQ, individual adjustment for markers of socioeconomic position rarely led to attenuation; indeed, positive confounding/mediation was apparent on several occasions (e.g. IQ vs. home and sporting accidents).
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In Table 4, we report on the relation between IQ score at 10 years of age and self-reported accidents requiring medical attention since the age of 16 in women. Some gradients were similar to those seen in men, while others differed. In women, as in men, there was no association between childhood IQ score and risk of having an accident as a pedestrian. The relation between childhood IQ score and risk of accident at home was in the same direction as that seen in men, but much weaker and not statistically significant (OR 1.05, 95%CI 0.95–1.16). Women with higher childhood IQ scores were also more likely to report having accidents while playing sports (OR 1.70, 95%CI 1.49–1.94) and in other, unspecified contexts (OR 1.26, 95%CI 1.08–1.47). Risk of experiencing a sporting accident was attenuated after adjustment, particularly for annual earnings, but remained significantly increased by 20% per SD increase in IQ score after multiple adjustment for all indicators of socioeconomic circumstances (OR 1.20, 1.01–1.44). Further control for the frequency with which study participants engaged in physical exercise at the time of the 30-year follow-up had little effect. The relation between IQ score and risk of having an accident in other, unspecified circumstances was changed little by adjustment for indicators of socioeconomic position. After full adjustment, a 1 SD increase in childhood IQ was associated with a 28% increase in risk of reporting this outcome (OR 1.28, 95%CI 1.04–1.58). In contrast to men, risk of having an accident while in a vehicle (OR 1.18, 95%CI 1.08–1.27) and while at work (OR 1.16, 1.04–1.29) was significantly greater in women who had higher childhood IQ scores (in men, these relationships were inverse). However, both these trends were markedly attenuated by adjustment for annual earnings, and, in the case of vehicle accident, by educational qualifications (OR 1.04, 95%CI 0.79–1.36). Further adjustment for car ownership weakened the relation between childhood IQ score and vehicle accident, such that confidence intervals included unity (OR 0.97, 95%CI 0.86–1.10). In general, with the exception of earnings, individual adjustment for other indices of socioeconomic circumstances did not have a marked effect on the IQ-accident association.
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In the analyses described above, we assessed the relation between adult reports of accidents and childhood general ability (g) which we extracted from a principal components analysis of scores on four tests of ability (matrices, digit recall, word definitions and word similarities) and transformed into an IQ score. To explore whether any aspects of childhood cognitive function were associated with accident risk after the effect of general ability was subtracted, we saved the residuals from linear regression analyses of each test on IQ score and examined the relation of each of them separately with accident risk. Men who scored higher on the non-g aspect of the digit recall test were less likely to report having an accident while in a vehicle (OR 0.87, 95%CI 0.79–0.96, after multivariate adjustment). Women who scored higher on the non-g aspect of the matrices test were more likely to report having an accident while playing sport (OR 1.27, 95%CI 1.04–1.55, after multivariate-adjustment). We found no other significant associations, suggesting that it is primarily childhood general ability (rather than any other specific aspect of cognitive function) that predicts risk of some types of accident in adult life in the present study.
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Discussion |
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Our aim was to examine the relationships between mental ability at age 10 years and self-reported accidents (from a range of causes) by age 30 years. These relationships were complex, differing according to sex and the type of accident under consideration. In both sexes, IQ score was positively related to the risk of sustaining accidental injuries while participating in sports and in other unspecified circumstances (after adjustment for markers of socio-economic position). In men only, higher childhood ability was associated with an increased risk of having an accident at home. Ability was positively related to accidents in the workplace in women, while in men this relation was in the opposite direction; both these associations were weakened by adjustment for markers of socio-economic circumstances. Higher childhood IQ score was associated with an increased risk of vehicle accidents in women (an effect not seen in men) but this relation was no longer apparent after adjustment for educational qualifications, earnings and car ownership. In the present analytical sample (rs=0.43; p < 0.001) and other studies,11 education and IQ were strongly positively correlated. Given that education may be a partial proxy for IQ, whether it should be included as a covariate in the statistical models is debateable.28 It is plausible that education may at least partially mediate the IQ-accident relation.
Comparisons with previous studies
While the number of studies examining the link between mental ability and chronic disease is growing,28–31 few investigators have examined the role of this psychological trait in the aetiology of accidents. Studies showing an inverse relationship between early life IQ and total mortality in participants followed to middle-age32–34 allude to an association between mental ability and accidents, given that the latter represents an important cause of death in this age group.1,28 As described above, motor vehicle accidents and offences have been most commonly linked to mental ability. In both cross-sectional17 and prospective studies,16,35 a higher risk of such events was seen in lower-IQ-scoring persons relative to their higher-performing peers. These effects appeared to be independent of socioeconomic position, in studies that took it into account.16,35 In the present study, childhood ability was not related to risk of accidents as a pedestrian, although the number of events was small. We found some evidence that higher childhood IQ score might have a protective effect against accidents as a passenger or driver in men, but this trend was not statistically significant at conventional levels. It seems probable that a stronger relation might have been evident in men if we had been able to carry out a separate examination of accidents that occurred while driving, rather than while being a passenger, but the study data did not permit this differentiation.
We are aware of only one study that has examined the link between mental ability and accidents other than those involving motor vehicles. In a cohort of children drawn from the Scottish city of Aberdeen and surrounding areas,35 (which, like ours, offered IQ scores across the full range) low test performance predicted an elevated risk of unintentional injury taking place on railways, and of injuries attributable to falls, poisoning and medical error, by middle-age; there was no apparent effect modification by sex.35 In that study, accidents were based on events serious enough to warrant hospital admission. In general, similarly consistent results are seen in studies relating markers of education (a close correlate of mental ability) to mortality due to accidents.5,6 By contrast, the relationship between education (and other markers of socioeconomic position) and self-reported non-fatal accidents is much less clear.8,9,36,37 While some self-reported accidents in the present study would have been severe in nature, others would have been more trivial. We had insufficient data on the seriousness of the injuries incurred in each accident to be able to test the suggestion that severe accidents had a more consistent association with mental ability.
Our finding of a positive relationship between IQ and non-fatal injuries sustained during sporting activities mirrors the association seen when socioeconomic position is the exposure of interest—in such studies, social advantage is associated with an increased prevalence.8,9 One explanation advanced for this gradient is that persons in the higher social groups are more likely to participate in leisure time physical activities than the disadvantaged, making the potential for injury greater.9 Using the same dataset, we have previously reported that childhood IQ is related to pattern of physical activity, with higher scorers more likely to take part in active leisure pursuits than their lower-performing peers.38 Adjusting for physical activity had essentially no impact on the relationship between IQ and sport accidents, though our data on physical activity related to exercise habits at the age of 30 and may not be an accurate reflection of the frequency of exercise at the time of the sporting accident. The stronger relation between IQ and home-based accidents in men rather than women may be because men engage more frequently in activities that might lead to such incidents—men have a 10-times higher risk of fatal accidents at home than women, while engaged in home repairs or improvements, car care, or gardening.39 The present dataset did not, however, allow us to examine this hypothesis in any detail.
Strengths and limitations
The strengths of this study are its size (resulting in high statistical power), its breadth of data on socio-economic position (which allow an examination of its role both as a confounding and as a mediating variable), and the representative nature of the study population (leading to high generalizability). Inevitably, however, there are some limitations. Firstly, its longitudinal nature has inevitably led to some attrition: only 46% of the participants at the 30-year follow-up had taken part in all earlier surveys of the cohort, although 80% had missed none or only one of these earlier sweeps, and 74% had taken the British Ability Scale tests as part of the 10-year follow-up.40 This group did have significantly higher IQ than those who did not take part in the 30-year follow-up, but the size of the differences was small (0.3 SD). Unless the relation between childhood mental ability and adult accidents is different in non-responders or those who have died, no bias will have been introduced. Secondly, the outcome variables were based on self-reported accidents recalled over a 15-year period. We are unaware of any studies validating these reports with those captured using other methods such as hospital admissions records. Thirdly, while we had data on accidents in adulthood (16–30 years), this information was not collected earlier in life.
In conclusion, in the largest study to date to examine these associations, the relation of childhood mental ability with adult self-reported accidents was complex, with gradients differential according to accident type and sex. Given the current paucity of data, further studies are required.
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The 10-year follow-up was carried out by the Department of Child Health, Bristol University. The 30-year follow-up was carried out under the auspices of the Joint Centre for Longitudinal Research (comprising the Centre for Longitudinal Studies, Institute of Education, University of London, the International Centre for Health and Society, University College Medical School, London, and the National Centre for Social Research). We thank the UK Data Archive, University of Essex, for providing the data. The original data creators, depositors or copyright holders, the funding agencies, and the UK Data Archive bear no responsibility for the analyses and interpretation presented here. DB is a Wellcome Fellow. ID is the recipient of a Royal Society-Wolfson Research Merit Award. IS is funded by UK ESRC grants L326253061, RES-225-25–2001, and RES-000–22–1748.
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  G. D. Batty, C. R. Gale, P. Tynelius, I. J. Deary, and F. Rasmussen IQ in Early Adulthood, Socioeconomic Position, and Unintentional Injury Mortality by Middle Age: A Cohort Study of More Than 1 Million Swedish Men Am. J. Epidemiol., March 1, 2009; 169(5): 606 - 615. [Abstract] [Full Text] [PDF]
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