Letters to the Editor

Dear Sir

Adeline More takes me to task (Newsletter, December 1994) and even accuses be of being a Marxist for suggesting that science had little impact on technology before the 19th century (report of 1993 Darwin Lecture, Newsletter, September 1994). All the evidence that I have come across supports my view. The steam engine, for example, owed very little to science and all the great renaissance buildings in no way relied on Newtonian mechanics. The one area where science did have an impact was navigation.

Lewis Wolpert

Dear Sir

C B Goodhart’s article on the heritability of IQ accepts twin studies as legitimate evidence for an ‘environmental’ cause of differences between identical (monozygotic) pairs reared apart, and so must the vast majority of those interested in the nature-nurture debate still continuing almost 20 years after the Burt affair began.

A matter that receives little attention is the X-linkage of IQ (Lehrke, 1972) which has the consequence that in all females the X-chromosomes are dosage-compensated (lyonised) at an early stage of development. In mosaic heterozygotes (about half of women) the differing cell types this produces are not thoroughly mixed at the time when identical twins separate. This was given as a conjecture by Maccoby & Jacklin (1974) and later confirmed by Osborne (1978) in his twin studies. Consequently female monozygotes are not always identical for their X-linked phenotypes. Evidence based on separated female ‘identical’ twins cannot be used and neither can data for separated identical twins including female pairs. The effect on correlation is small (0.02-0.09) but persistent, applying to most mental tasks.

Much more disturbing is the following contingency table, which has recently emerged during preparation of papers on a survey of more than 4000 Moray House Test (verbal IQ) results for Orkney schoolchildren tested in the period 1947-76. Normally the test is administered twice - at age 10±1 years and at 11±1 years, the second result generally being accepted as more reliable. Only 23 pairs of twins are found; a low yield of mixed monozygotic and dizygotic pairs who cannot be classified.

The temporal pattern of death and mixed tests is the same for twins and singletons, most being congregated in the early years of the survey. Those missing tests have been retarded in their schooling through protracted absence, generally caused by complications of ‘ordinary’ childhood diseases such as German measles or chicken-pox. All five of the twin deaths were boys. Those missing tests are not pairs of twins, but single individuals from twin pairs. Evidently a factor acts divisively in producing these results.

To be sure, the factor must indeed be environmental. All these twins were reared in the family home. Lack of educational opportunity is certainly not the cause of the differences. The matter is one of ill-health with a very strong presumption that the immune system is affected in one twin and not the other. The effect is intra-uterine, one twin faring better than the other in many cases.

It seems the question should invariably be asked where separated twins are concerned: “Why were they ever separated?”. Should the answer be “Because the parents were unable to rear children”, then intra-uterine privation acting divisively becomes highly probable.

Older twin data such as Burt’s are highly suspect, for there have been substantial improvements in ante-natal care and procedures during the past 30 years. Even so, parents unable to rear children are an “at risk” group liable not to profit from such care.

Roger B Anderton

Lehrke, R (1972) A theory of X-linkage of major intellectual traits. American Journal of Mental Deficiency. 76, 611-619.

Maccoby, E E & Jacklin, C N (1974) The psychology of sex differences. Stanford University Press.

Osborne, R T (1978) Race and Sex Differences in Heritability of Mental Test Performance: A Study of Negroid and Caucasoid Twins. In: Human Variation: The Biopsychology of Age, Race and Sex. Edited by Osborne, RT, Noble C E & Weyl, N. Academic Press, New Yory, San Francisco, London.