Olkkola, Maarit (2014). 'Poor Cognition – Early-life Socioeconomic Status and Cognitive Abilities in Adulthood, The Helsinki Birth Cohort Study 1934–1944' Paper presented at the annual conference of the HDCA, 2-5 September 2014, Athens, Greece.
Introduction. Cognitive abilities are associated with success in many aspects of life: according to several studies more cognition translates to more health, education and income. Persistent differences in cognitive abilities have been detected in childhood and in many cases have been associated with family socioeconomic status (SES). Cognitive capacity is thus a key structural reserve for capability development and human capital accumulation.
Aim. The aim of this study is to examine the effects of early life SES on cognitive abilities in early and late adulthood in a historical context where SES was likely to translate into substantial differences in the developmental environment of a child.
Theoretical background. Research under the Developmental Origins of Health and Disease (DOHaD) paradigm and advances in epigenetics and neuroscience have provided a body of evidence linking the early life developmental environment to differences in brain development. Research in both humans and animal models has identified several mechanisms affecting brain development in critical periods leading to changes in gene expression, brain physiology and cognition. Many of these critical periods occur in fetal and infant life. Some exposures can lead to irreversible structural changes in the brain but some are reversible, depending on further exposure. Nutrition, in particular, seems to be important, since an infant uses up to 40 per cent of its energy for growth during the first two years of life.
In this study, risk factors for cognitive development are hypothesized to cluster in the lower end of the socioeconomic status spectrum whereas protective factors are hypothesized to cluster in the higher end. These cumulative exposures are hypothesized to affect the epigenetic regulation of gene expression and to exacerbate underlying genetic vulnerabilities more in the lower SES groups than in the higher groups. In epidemiological studies some epigenetic marks have been associated with cognitive abilities and with socioeconomics status, respectively, but this evidence is still tentative.
In this cohort it has been previously shown that birth weight and length, head circumference at birth and growth trajectories in infancy and childhood are associated with performance in cognitive tests at approximately age 20 and 68, respectively. It has also been shown that maternal hypertensive disorders during pregnancy and separation from parents during World War II are associated with the test performance. In these studies, early-life socioeconomic status as measured by father's occupational status has been shown to correlate strongly with test performance. The test results have been shown to be a good predictor of later-life educational and occupational success and these associations are not explained by father's occupational status.
Methods. A directed acyclic graph (DAG) will be created to specify the hypothesized theoretical pathways between socioeconomic status, the mediating variables and cognitive abilities. Based on this theoretical model, a statistical structural equation model (SEM) will be created and tested with empirical cohort data. This model will include measured mediating variables, such as birth weight, length of gestation, head circumference, infant growth and breastfeeding status, as well as unmeasured variables such as nutrition, cognitive stimulation, toxic exposures and parents' cognitive abilities. The hypothesis to be tested is that early life socioeconomic status is systematically related to later life cognitive abilities through the mediating variables.
Early-life socioeconomic status variables will be constructed with information collected form the Helsinki municipal taxation records. Both parents' income based on paid taxes and a historically based categorization of occupational status will be used. The cognitive ability measurements come from the Finnish Defense Forces basic ability test. It is composed of verbal, arithmetic, and visuospatial reasoning subtests, each consisting of 40 timed multiple-choice questions.
The Helsinki Birth Cohort includes 13345 individuals born in either of the two public hospitals in Helsinki between 1934 and 1944. 4630 men from this cohort had birth and child welfare clinic records available and were still residents of Finland in 1971, when a unique personal identification number was allocated to each Finnish citizen. 2786 of these men had served in the Finnish Defense Forces between 1952 and 1972 and had taken a compulsory cognitive abilities test during the first 2 weeks of their military service at an average age of 20.1 years. In 2009, the same test was retaken by 931 of these men at an average age of 67.9 years.
There are some differences in the characteristics of the men in the original cohort and in the subsamples and these differences will be taken into account by a non-participant analysis. The final sample will slightly decrease further because the parents' tax records are collected only for those who lived in Helsinki during their first years of life. It is likely that some of these men did not live in the municipality of Helsinki but in the surrounding municipalities, because Helsinki only expanded considerably after 1945. Key strengths of this study are the follow-up period of seven decades and the fact that both anthropometric and socioeconomic measures are based on reliable recorded data, not on recalled values.
Discussion. There is ample evidence from epidemiological studies to show that the developmental environment of a child has long-term effects on later-life physical and mental health, including cognitive abilities. This has important implications for human development theory and social policy, especially in developing countries. The lifelong consequences of the developmental environment emphasize the importance of investments in prenatal and childhood health in order to avoid creating structural barriers to further capabilities development. It is also essential to consider the complementarities of cognitive abilities with other investments, such as education, when evaluating the costs and benefits of these investments.