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Rapid Responses: Rapid Responses published:
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Mild learning disabilities within the general population
- Woody Caan (3 February 2006)
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IQ and occupational exposures
- Jean-Francois Gehanno, Joel Ladner (13 March 2006)
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Intelligence and health
- Tom Hughes-Davies (14 March 2006)
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Mortality and intelligence in the West of Scotland twenty-07 study
- Archana Singh-Manoux, Jane Ferrie, UCL (18 March 2006)
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Racism explains the socioeconomic inequalities in health
- Dipen Rajyaguru (21 March 2006)
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Research on relevance of IQ to socioeconomic inequalities in health does not contribute to more successful strategies to tackle health inequalities
- Per Carlson, Sarah Wamala, PhD. Head of Unit and Associate Professor (22 March 2006)
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IQ at 56 and subsequent mortality in the West of Scotland Study
- Denny H Vågerö (26 March 2006)
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Re: Research on relevance of IQ to socioeconomic inequalities in health does not contribute to more successful strategies to tackle health inequalities
- Mel Bartley (15 January 2008)
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Woody Caan, Professor of public health Anglia Ruskin University, Chelmsford CM1 1SQ, UK.
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We are indebted to this Medical Research Council study of intelligence and adult health [1], which expands on the social gradient of intelligence most elegantly described by Marmot [2] across adults in regular, paid employment. However, these authors assume that in a community sample, IQ is normally distributed and that any effects on health measures operate in a consistent manner across that distribution. This is not true for the asymmetrical, left part of the IQ curve, corresponding roughly to the 3% of the population with learning disabilities. Most of these people have mild learning disabilities, of the sort associated with low birth weight, premature delivery, maternal binge alcohol use or chronic maternal infection [3]: that is with impaired foetal and neonatal development that past MRC research has so strongly associated with poor adult physical and mental health. When the older subjects of Batty et al [1] were growing up, early mortality especially death from coronary heart disease, was associated with a learning disability [4]. As reported in the BMJ, we do not know whether a small number of developmentally disadvantaged individuals with the lowest IQ scores contributed to the observed 'marked reduction' in other social gradients for health outcomes, or whether people with learning disabilities did not figure significantly in this research. However, in public health action to address inequalities, proactive 'health facilitation' to improve the life chances of young people with disabilities may prove a good investment [5]. 1 Batty GD, Der G, Macintyre S, Deary IJ. Does IQ explain socioeconomic inequalities in health? Evidence from a population based cohort study in the west of Scotland BMJ 2006; 0: bmj.38723.660637.AEv1 2 Marmot M. Status Syndrome. How your social standing directly affects your healh and life expectancy. London: Bloombury, 2004. 3 Caan W. Being of sound mind, in the beginning… Department of Health: Mental Health Promotion Update 2005; 2: 13-15. 4 Caan W, Hampton-Matthews S. One from the heart, for people with a learning disability. British Journal of Nursing 1999; 8: 97-100. 5 Caan W, Lutchmiah J, Thomson K, Toocaram J. Health facilitation in primary care. Primary Health Care Research & Development 2005; 6: 348- 356. Competing interests: Engaged in research projects on the health of young people with special needs, in the community. |
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Jean-Francois Gehanno, Occupational Physician Rouen University Hospital, Rouen, 76000, France, Joel Ladner
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In their paper, David Batty and Colleagues have made an important contribution to our understanding of the relations between IQ and socioeconomic gradients in health. Nevertheless, we think that the relations between socioeconomic status and IQ should be considered. Yet, although it is assumed that low intelligence might be a cause for social class inequalities in health, some data demonstrate that low IQ could be an indirect consequence of poor socioeconomic status and of occupational exposures. Many studies show clear relationship between risk factors in parents' prenatal occupational histories and subsequent mental retardation in the offspring. Such a relationship has been demonstrated for women's prenatal employment maternal employment in the textile and apparel industries1 or for women exposure during pregnancy to ionizing radiation2, lead3, mercury or organic solvents4, among others. Since the most socially disadvantaged are usually over-represented in workers exposed to such occupational risk factors5, a link between socioeconomic status and low IQ in the offspring can be assumed. These data have implications on the interventions aimed at reduce socioeconomic inequalities in health. Efforts to reduce pregnant women's occupational exposures should also be made and this should contribute to reduce such inequalities. 1. Decoufle P, Murphy CC, Drews CD, Yeargin-Allsopp M. Mental retardation in ten-year-old children in relation to their mothers' employment during pregnancy. Am J Ind Med 1993;24(5):567-86. 2. Vos O. Effects and consequences of prenatal irradiation. Boll Soc Ital Biol Sper 1989;65(6):481-500. 3. Fewtrell LJ, Pruss-Ustun A, Landrigan P, Ayuso-Mateos JL. Estimating the global burden of disease of mild mental retardation and cardiovascular diseases from environmental lead exposure. Environ Res 2004;94(2):120-33. 4. Roeleveld N, Zielhuis GA, Gabreels F. Mental retardation and parental occupation: a study on the applicability of job exposure matrices. Br J Ind Med 1993;50(10):945-54. 5. Boffetta P, Kogevinas M, Westerholm P, Saracci R. Exposure to occupational carcinogens and social class differences in cancer occurrence. IARC Sci Publ 1997;(138):331-41. Competing interests: None declared |
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Tom Hughes-Davies, Paediatrician Breamore Marsh, Fordingbridge SP6 2EJ
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Levels of lead exposure within accepted limits may profoundly affect intelligence. It would be odd if such insults affected only cells concerned with intelligence. Canfield et al (1) reported 166 children with life time averages under 20 micrograms lead /dl blood, half of them under 10. At 5 years their average intelligence quotient was 90. Eleven had an IQ over 120. All but one of these had a lead level below 5. A small shift in the mean of a bell shaped curve masks large changes in incidence on its steeper slopes. 1. Canfield RL, Charles R. Henderson CR, Deborah A. Cory-Slechta DA et al. Intellectual Impairment in Children with Blood Lead Concentrations below 10 µg per Deciliter. New Engl J Med 2003;348:1517-1526 Competing interests: None declared |
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Archana Singh-Manoux, Chargée de Recherche INSERM U687 FRANCE 94415, Jane Ferrie, UCL
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The interrelationship between intelligence (or cognitive ability), socioeconomic position and health is a fascinating one; the paper by Batty and colleagues [1] is important in the attempt to quantify some of these relationships. The view that intelligence is the fundamental cause of social inequalities sees cognitive ability to be the driving force behind both socioeconomic attainment and health. The alternate view is that although socioeconomic position and cognitive ability are related to each other, social inequalities in health cannot be explained by group differences in intelligence. In a recent paper on data from the Whitehall II study, we have shown the constructs of health and socioeconomic position to be multifaceted; not all health outcomes show the same social patterning and not all measures of socioeconomic position are similarly related to health [2]. The replication of our analyses by Batty and colleagues shows similar results in the West of Scotland twenty-07 study, despite obvious differences in the population examined. Thus, it appears that even though cognitive ability is related to health, it does not explain social inequalities in health. In addition to the outcomes explored in our paper, Batty and colleagues were able to examine total mortality and coronary heart disease mortality. Their results show that intelligence explained considerably more of the socioeconomic gradient in both all-cause- and coronary heart disease mortality. Why?... It is possible that this finding will be replicated in other studies and lead to the conclusion that intelligence predicts mortality better than it predicts health itself. However, it is also possible that the answer lies in the characteristics of the West of Scotland twenty-07 study. In this sample of 1347 middle aged individuals (mean age 56, SD= 0.6), nearly a third (26.9%) died in the 16-year follow up of the study. The WHO health indicator statistic (2002) for the UK reveals a 'healthy' life expectancy at age 60 to be 15.7 years for men and 18.1 for women (http://www3.who.int/whosis/country/indicators.cfm?country=gbr). Thus, the mortality rate in the West of Scotland twenty-07 study must exceed national rates. Is it possible that the finding related to mortality, that intelligence explains more of the socioeconomic gradient, is specific to socially deprived populations? One way of exploring this hypothesis would be to examine the prevalence rates of all outcomes (mortality in particular) as a function of the indicators of socioeconomic position and intelligence. If the mortality rate during follow-up is considerably higher at the bottom of the social hierarchy or among those with low intelligence scores, the analysis undertaken by Batty & colleagues is problematic. The calculation of a relative index of inequality (RII) requires assumptions of linearity between the predictor and the outcome variable. There is some evidence to suggest that intelligence and mortality are not related to each other in a linear fashion. Results from the MRC National Survey of Health and Development study show that in men excess mortality during follow-up was concentrated in the bottom quarter of the cognitive score and in women there was no association between intelligence and mortality [3]. Could something similar be happening in the results reported by Batty and colleagues? Using the RII can be problematic in these circumstances. Archana Singh-Manoux Chargée de Recherche INSERM U687 HNSM, 14 rue du Val d'Osne 94415 Saint-Maurice Cedex France Jane Ferrie Senior Research Fellow Department of Epidemiology & Public Health University College London London WC1E 6BT UK 1. Batty GD, Der G, Macintyre S, Deary IJ. Does IQ explain socioeconomic inequalities in health? Evidence from a population based cohort study in the west of Scotland BMJ 2006; 332: 580-40. 2. Singh-Manoux A, Ferrie JE, Lynch JW, Marmot M. The Role of Cognitive Ability (Intelligence) in Explaining the Association between Socioeconomic Position and Health: Evidence from the Whitehall II Prospective Cohort Study. Am. J. Epidemiol., 2005; 161: 831-839. 3. Kuh D, Richards M, Hardy R, Butterworth S, Wadsworth ME. Childhood cognitive ability and deaths up until middle age: a post-war birth cohort study. Int J Epidemiol. 2004; 33: 408-13. Competing interests: None declared |
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Dipen Rajyaguru, Health Equality & Diversity Specialist Barnet PCT
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With regard to the research, I believe it starts from a flawed assumption that IQ test show intelligence. Plenty of research and a bit of common sense tells us that people from Black & Minority Ethnic (BME) communities do not do well on IQ test because they are Eurocentric in the methods, thoughts and langauge. The assumption from the research could lead to racist stereoypes that we need to steer well clear from, such as the fact that the majority of BME communities tend to live in the most deprived boroughs of the country and suffer from the greatest inequalities in health does not make them stupid! A reason that goes further to explain the socioeconomic inequalities in health is simple racism, no matter how academic or scientific we get. You get two people from the same socioeconomic background, one white the other black and then measure the level of service, treatment and access each is offered. A racist in whatever profession or industry is a racist and it is their perception that prevents Black people from progression not IQ. Research such as this although conducted in West Scotland will be used to stigmatise whole BME communities. This study does not and cannot explain why particularly students from South Asian communities do well at school and better than their white counter-parts nationally, but are still from the poorest and most deprived neighbourhoods. Competing interests: None declared |
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Per Carlson, PhD. Research officer Swedish National Institute of Public Health, SE-83140 Östersund, Sweden, Sarah Wamala, PhD. Head of Unit and Associate Professor
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Batty, et. al. concluded in their cross-sectional study that IQ substantially attenuated socioeconomic inequalities in morbidity and mortality. There are some critical issues that should have been addressed in this paper. First, the items that were included in the IQ measure seem to have a high correlation with educational attainment particularly in this group of elderly participants. It is not surprising that the reduction in RII is over 100% for all cause and CHD mortality (Table 1). Second the lifecourse socioeconomic influences on IQ should have been given more attention in the discussion. It is more plausible that individuals who grow up in families with socioeconomic disadvantage develop lower IQ, which may be observed even in future generations. Thus IQ in itself may not create socioeconomic inequalities rather socioeconomic inequalities are more likely to create differentials in IQ in the population. If the aim of the study was to test the hypothesis that IQ is a fundamental cause of socioeconomic inequalities in health, then a discussion on more convincing mechanisms would have been useful. A more thorough discussion on why certain socioeconomic groups have lower IQ than others was necessary, but was avoided in the paper. Third, statistical adjustment for sex does not tell us how socioeconomic inequalities in morbidity and mortality are modified by IQ in men and women. It is plausible that the contribution of IQ to social inequalities in health may vary by gender. In conclusion, socioeconomic inequalities in health are a public health issue and strategies to tackle these inequalities need to have a public health perspective. Thus results on IQ’s significance on socioeconomic inequalities, when not properly explained, do not offer any alternatives for public health interventions to reduce social inequalities in health, rather, they contribute to peoples´ prejudices on why some groups in society are healthier than others. So it is the individuals to blame. Per Carlson, PhD. Research officer Sarah Wamala, PhD. Head of Unit and Associate Professor Swedish National Institute of Public Health, Stockholm, Sweden Competing interests: None declared |
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Denny H Vågerö, professor, director Centre for Health Equity Studies, CHESS, Stockholm University/Karolinska Institutet, SE-106 91
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Batty and colleagues show that cognitive abilities at age 56 predict health at the same age as well as mortality in a 16-year follow-up. This is an important addition to a number of recent studies in this emerging field of research. I would like to add the following comments and questions about how we should interpret their findings. 1. IQ was measured at age 56. In line with ideas in a classic paper by Paul Baltes [1], we should assume that intelligence changes with age and aging. Shu-Chen Li and colleagues [2] suggest that this starts well before age 56 [2]. In their views this is particularly the case for “fluid” intelligence and less the case for “crystallized” intelligence. The former may be more closely linked to brain physiology while the latter may be more closely linked to education and experience. Thus, at age 56 measured IQ has probably fallen in response to aging and disease load, both of which are socially differentiated. If controlling for IQ is “partly controlling for sub-clinical health status”, as the authors themselves suggest, how important is this? 2. If cognitive ability changes over the life span, surely this change is not independent of how the social circumstances of individuals change? 3. Discussing potential explanations, the authors conclude: “IQ may be a record of bodily insults across the life course”. To me this seems a fruitful and attractive idea compatible both with Baltes [1] and with results in this paper. However, it seems hardly consistent with the “novel hypothesis” that IQ might be “the elusive fundamental cause of social class inequalities in health” that is put forward in the introductory section. 4. In spite of recent findings (3) the biological basis of IQ may in fact be more elusive than are the social determinants of health. Denny Vågerö Professor, director Centre for Health Equity Studies, CHESS Stockholm University/ Karolinska Institutet denny.vagero@chess.su.se [1].Baltes P. B. Theoretical propositions of life-span developmental psychology: On the dynamics between growth and decline. Developmental Psychology 23:611-626, 1987. [2]. Li S-C, Lindenberger U, Hommel B, Aschersleben G, Prinz W, Baltes PB. Transformations in the couplings among intellectual abilities and constituent cognitive processes across the life span. Psychological Science 15:155-163, 2004 [3]. Colom R, Jung R, Haier R (2006). Distributed brain sites for the g- factor of intelligence. Neuroimage doi:10.1016/j.neuroimage.2006.01.006 Competing interests: None declared |
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Mel Bartley, Professor of Medical Sociology Univeristy College London, WC!e 6BT
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Dear colleagues, One requirement for adequate explanations in the study of population health is that explanations for differences between groups (ethnic, gender, occupational, etc) should be consistent with differences within groups over time. Since the 1970s, mortality rates in men of middle age and above in the UK have fallen by almost half. Other readers may be aware of data on changes in measures of cognitive performance over this time. But as the difference between social classes is rather smaller than the difference between time periods for the whole population, we would certainly need a 'fundamental cause' that was consistent with the time trends. If we could better understand these trends, policies to improve population health would be a great deal strengthened. Yours sincerely Mel Bartley Competing interests: None declared |
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