The Association Between Birth Weight and Cognitive Function and of Childhood IQ and Survival

Sir Isaac Newton told John Conduitt, husband of his half-niece, Catherine Barton, that, when he was born on Christmas Day 1642, ‘he was so little they could put him into a quart pot & so weakly that he was forced to have a bolster all round his neck to keep it on his shoulders’, and thought unlikely to survive¹. He was undoubtedly a diminutive neonate, with a low birth weight more likely to have been due to poor, or improper, nourishment while in utero than due to premature birth by more than a few days and still have survived in the seventeenth century. He was short of stature at five feet six inches tall (the same as Beethoven and Napoleon), and the statue of him in the Chapel of Trinity College, Cambridge gives him a small head, which Roubiliac had sculpted using his death mask for size.²

When bones from 16 important clerics – likely because of their ecclesiastical ranks to have been well nourished – buried at Ely Cathedral in the tenth and eleventh centuries were examined, it was estimated that all of them had been between five feet ten inches and six feet two inches tall, which latter height was also that of Henry VIII. There is little genetic indication for populations of short height in immediate past centuries, but every indication, according to J M Tanner³, that malnutrition, particularly in early childhood, prevents a population attaining the full height that its genetic endowment might have expected.

The earlier in life the malnutrition, the longer lasting its effects tend to be, so a very small baby is likely to fail to catch up and attain its full height potential. When there is extreme malnutrition in early life, though the growth of the brain is largely spared, the head circumference may be less than it should be, and intelligence may follow suit⁴. This is of particular interest with regard to Isaac Newton, though he must be the exception, of course, with regard to his intelligence! Intrauterine growth retardation, such as that due to inadequate placental circulation, has been discussed by Levene and Dubowitz⁵.

The poor cognitive development associated with small size at birth is not dependent on the family environment, such as socioeconomic status and birth order⁶, and most studies have focussed on clinically low birth weight rather than on the full birth weight range in the normal population. A longitudinal study has recently been published in the British Medical Journal on 3900 males and females born in one week in March 1946, that examines the relationship between birth weight and cognitive function⁷. Information about sociodemographic factors and medical, cognitive, and psychological function was obtained at ages 8, 11, 15, 26 and 43 years. This allowed the investigation of relative change of cognitive function according to birth weight split into five categories, with the association between birth weight and educational attainment also being assessed.

In the general population, birth weight was significantly and positively associated with cognitive ability at age 8 between the lowest and highest birth weight categories. This was after sex, father’s social class, mother’s education, and birth order were controlled for. The association was evident across the normal birth weight range and so was not accounted for exclusively by low birth weight. It was also observed at ages 11, 15, and 26, and weakly at age 43, although these associations were dependent on the association at age 8. Birth weight was also associated with education, with those of higher birth weight more likely to have achieved higher qualifications, suggesting that the association between birth weight and cognition may have functional implications.

There has been speculation about how the association between birth weight and cognitive function might occur⁸. There is evidence indicating that insulin-like growth factors (IGFs) play a critical role in determining overall (somatic) body growth in addition to contributing to local tissue regulation.⁹ IGFs are peptides that regulate the growth, metabolism, survival, and differentiation of cells and are regulated by growth hormone. It has been found that early in life IGFs and growth hormone play a role in the development of parts of the brain that are responsible for learning and memory, which could then explain associations between body size and subsequent measures of cognitive functions. It is more difficult to explain how circulating IGFs might be connected with cognitive function in later life.

Another longitudinal study¹⁰, recently published, has been to test the association between childhood IQ and survival up to age 76. Inequalities in health and mortality exist among different socioeconomic groups^11,12,13, besides the health of adults with differing socioeconomic conditions in childhood¹⁴. Educational level also contributes to differences in mortality between the socioeconomic groups¹⁵. Socioeconomic status, educational level, and mental ability are closely linked. However, there is little information about the link between mental ability and morbidity and mortality.

79.9 per cent of 2792 children born in Aberdeen in 1921, who sat a mental ability test in 1932, were traced, and both males and females who died before 1997 had a significantly lower mean IQ at age 11 than those who were alive or untraced. Regression analysis showed childhood mental ability was positively related to survival to age 76 in both men and women, and is a significant factor among the variables that predict age at death. The influence of childhood IQ on survival was weaker in men than in women, but this could have been due to the effect of the second world war on death rates in men. Controlling for overcrowding did not alter the association between mental ability and mortality.

Robert Plomin (in an interview reported in The Times , 27 July 1998) has found the issue of the environment to be even more complex than imagined, and has renamed the disparate influences of nature and nurture the “nature of nurture”. He points out how children brought up in the same home do not necessarily share the same environment, as parents often treat children differently in reacting to a child’s behaviour. The family environment may be affected by genes, he suggests, with the family members choosing or shaping their surroundings according to their genes. [Retrospectively it is easy, I find, to acknowledge the likelihood of all this when pondering on one’s own parents’ family reactions.] In addition, a child’s differing experiences outside the home may be significant in moulding character.

The paper on the association of birth weight with cognitive function and the study showing that a normal life span is related to childhood IQ bear out Plomin’s observation that the environment is even more complex than ever imagined. In thinking of it I have always, it seems, thought of its influences as only possibly beginning at birth except in very rare circumstances. But now, in learning about the association of birth weight with cognitive function, the influence of ‘nurture’ must be thought of as starting from conception and, on discovering that survival is related to childhood mental ability, as persisting to death.

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