Department of Maternal and Child Health, School of Public Health, CB#7445 Rosenau Hall, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7445, USA. E-mail: Pierre_Buekens{at}unc.edu
SirsI was surprised to read the Commentaries that followed Allen Wilcoxs reformulations of strategies to analyse birthweight distributions.14 The Wilcox approach is not focussed on the origin of the variations in birthweight, but rather on the linkor absence of a linkbetween birthweight and mortality. It separates birthweight distributions into two components, a main and a residual distribution. The main Gaussian distribution corresponds to term babies, and its shift has no influence on mortality. Richard David suggests that using this approach to study birthweight among different races or ethnic groups is problematic because it may imply that some groups of babies are just supposed to be small.2 However, the shift of main birthweight distribution observed among different populations can be entirely explained by environmental factors and does not imply genetic determinism.
Race and ethnicity are social constructs, and reflect social realities, such as racism and discrimination.5,6 Numerous studies using the Wilcox approach have shown that the main birthweight distribution is strongly influenced by environmental factors, such as altitude, smoking, or social class.1,7 Differences among races and ethnic groups should be interpreted as another example of influence of environmental factors on birthweight distributions. An important contribution of Wilcoxs analysis of birthweight-specific infant mortality is that it unveils the high mortality of small African American infants.1 The appearance of better survival among small African American infants compared to white infants disappears after transforming all birthweight distributions into z-scores. Using British data, Wilcox found similar results for low social class infants.7 Finding solutions to these social disparities should be our priority, and building a consensus about our interpretation of the causes and consequences of birthweight distributions is a necessary step toward that goal.
References
1 Wilcox A. On the importanceand the unimportanceof birthweight. Int J Epidemiol 2001;30:123341.
2 David R. Commentary: Birthweights and bell curves. Int J Epidemiol 2001;30:124143.
3 Hertz-Piciotto I. Commentary: when brilliant insights lead astray. Int J Epidemiol 2001;30:124344.
4 Wilcox A. Response: where do we go from there? Int J Epidemiol 2001; 30:1245.
5 Cooper R, David R. The biological concept of race and its application to epidemiology. J Health Politics Policy Law 1986;11:97116.[ISI][Medline]
6 Wise P. Confronting racial disparities in infant mortality: Reconciling science and politics. Am J Prev Med 1993;9S:716.[ISI][Medline]
7 Wilcox A. Birthweight and Perinatal Mortality [Dissertation, University of North Carolina]. Ann Arbor, MI: University Microfilm International, 1979.