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Nigeria,1 analysed the attitudes of married women between the agesof 15 and 50. The first question was on the size of the family living atthe time of the survey. Women who had passed childbearing agereported an average of four surviving children each. All age groupswere asked what they thought to be the ideal number of survivorsand all women (including those who were still to bear children andthose who had finished) opted for about five survivors. This answermight well have fitted the views on family size prevalent in the Westnot so very long ago. Our grandparents would probably have beenshocked at the very idea of zero population growth. Five is a numberof children geared to the village, extended family way of life. Yourchildren are an insurance against old age; they are labour to continuethe farm work and even expand it. It is a figure which belongs to aconcept of unlimited space and a sparse overall population. But thetruth is that even at four children per woman there is a continuinggrowth in the West African population far beyond the needs of thefamily farm.The real horror of these figures, though, is that they are allied to

the simple fact that the women in the older age groups had to have atleast ten babies to achieve four survivors. A second horror is thateven these figures have produced a population increase which inNigeria has conservatively been estimated at 3% per annum. Thepopulation of the country has at least trebled since my first exposurein 1947. The health problems are soluble. Infant death has

readily identifiable components, including malaria, gastroenteritis,kwashiorkor, marasmus, and the diseases Meegan listed. But onecannot help ponder the consequences of a medical revolution. Thepopulation expansion might take on terrifying dimensions. Thesocial attitudes and mores behind the huge birth rates are deep andcannot be dealt with by simple birth control propaganda. Educa-

tion-but how?The problem solved itself in the West with no help from the plan-

ners-but it took more than a century of exposure to a highlyorganised technological bureaucracy in sparsely populated ter-

ritories before the economic and social virtues of small families tookcontrol of events. Herein lies my consolation. No-one, even fiftyyears ago, would have predicted the present low birth rates and highstate of infant health which prevails in North America and Europetoday. Although the media continue to present a hysterical pictureof health hazards on all sides, the truth is that never in the history ofman has there lived so healthy a people as the inhabitants of the so-called developed nations. It is my hope that the peoples of the ThirdWorld may be permitted to achieve a similar success under the in-fluences of natural and economic pressures. My fear, though, lies inthe questions-is there time, is there space?Department of Physiology,University of Toronto,Toronto, Ontario, Canada JOHN GRAYSON

TOXIC CHEMICALS IN THE THIRD WORLD

SIR,-The report of an outbreak of polyneuropathy due to tri-orthocresyl phosphate (TOCP) in Sri Lanka (Jan. 10, p. 88) is, Ibelieve, representative of a more widespread problem in Sri Lankaand perhaps in many other developing countries "benefiting" fromwestern technology.During a recent visit to Sri Lanka I learned that sporadic examples

of toxic neuropathies due to TOCP and various pesticides arethought to be common, especially in rural areas. The condition wasfatal in two relatives of a friend; two nieces survived.

Suppliers of toxic chemicals to developing countries bear anespecially heavy responsibility in ensuring that at every stage in thesupply chain to the user the hazards are known. In a setting of ig-norance and illiteracy, the enactment and enforcement of safetylegislation may also be inadequate.Hospital for Sick Children,London WC1N 3JH JOHN WILSON

1. Uyanga J. Economic determinants of urban family size: a case study of Calabar. NigerianBehav Sci J 1978; 1: 5-16.

EXERCISE AND THE HEART: ROUND TWO

SIR.-In their reply to the Jan. 3 correspondence on their article’Professor Morris and his colleagues (Jan. 31, p. 267) state "In factour data indicate that few men give up VE [vigorous exercise] inmiddle age for any reason; it is in their 20s and 30s that many slip intobad habits" (my italics). If this is how they approach their data, thenless puritanical readers may be forgiven a little scepticism!The hard evidence in their report is that in men reporting recent

VE the mortality rate from coronary heart disease (CHD) is I - 17ocompared with 2-9% of the remainder. That 1-1% represents 24men and when they add in those VE men with non-fatal disease(epidemiologically "softer" evidence) it amounts to 66 men. Thus,in dichotomising their data to look at possibly confounding variablesthey have a problem of small numbers and are careful to omitstatistical tests from most of their comparisons, relying on con-sistency of direction of difference to support their thesis. "Moreelaborate multivariate analysis", they state "is beyond the scope ofthis report".’ In my previous letter I pointed out that their resultscould be due to confounding variables, in particular (but not only)toselection out of the previous universe ofVE men because of existingCHD. Morris et al. reply that simple arithmetic proves this to be im-possible. I have done some simple arithmetic of my own, using dataon ECG abnormalities in their sample survey, where they reportedtwice the frequency of ECG abnormalities in the non-VE men. Us-ing the 71/2 year CHD mortality rates for normoglycaemic men with

. or without ECG abnormalities reported for the Whitehall Survey,3I found that the expected relative risk for non-VE men in the studyof Morris et al. would be 1 - 16 or 1 · 23, depending on which ECGcriteria in ref. 2 one uses. This is less than the relative risk of2’64

reported by Morris et al.,’ but it does illustrate the point that thenon-VE men started with an added risk. How this would affect their

comparisons if included in a multivariate analysis with other indicesof risk can only be guessed.Epidemiology is all about associations and their interpretation. I

accept that Morris and his colleagues, and others, havedemonstrated an association between reported and subsequentrisk of CHD. I do not accept that a causal hypothesis has beenestablished nor do I accept the notion that the debate is solely bet-ween that and the constitutional hypothesis. There are still manyconfounding variables to be sorted out before the debate canproperly begin. These considerations are well discussed in a

previous article by Morris et al. When did honest doubt turn tocertainty? 1,z,4Department of Community Medicine,Guy’s Hospital Medical School,London SE1 9RT R. J. JARRETT

***Professor Morris and colleagues were invited to reply.-ED. L.

SIR,-Dr Jarrett seems preoccupied that we have not given dueat-tention to the possibility that the non-exercising men will be loadeddisproportionately with the unfit, so accounting both for theirphysical inactivity and increased risk of heart-attack. Now somesuch self-selection obviously must occur, and as we have recalled(Jan. 31, p. 267), workers in this area have been grappling with theissue ever since the hypothesis on exercise and CHD was first pro-posed many years ago. We approached the problem in a variety ofways, analysing the VE and no-VE men (a) by the standard "risk fac-tors" for CHD, (b) for the presence of subclinical and overt CHD, (c)as they are sick or well in other respects, (d) for other health-relatedbehaviours and for attitudes to health, and (e) for numerous possiblyconfounding personal and social advantages and

disadvantages. 1,2,4,S None of this, singly or in sundry combinations,

1. Morris JN, Everitt MG, Pollard R, Chave SPW, Semmence AM. Vigorous exercise inleisure-time: Protection against coronary heart disease. Lancet 1980; ii: 1207-10

2. Epstein L, Miller GJ, Stitt FW, Morris JN. Vigorous exercise in leisure-time, coronaryrisk factors, and resting electrocardiogram in middle-aged male civil servants BrHeart J 1976; 38: 403-09

3. Fuller JF, Shipley MJ, Rose G, Jarrett RJ, Keen H Coronary-heart-disease risk and im-paired glucose tolerance- the Whitehall Study. Lancet 1980, i: 1373-76.

4 Morris JN, Chave SPW, Adam C, et al. Vigorous exercise in leisure time and the in-cidence of coronary heart-disease. Lancet 1973, i: 333-39