The Frequency of Twin Births in France: The Triple Influence of Biology, Medicine and Family Behaviour
Pison, Gilles, Couvert, Nadège, Population
The biological dimension may sometimes appear to be the determining-or even exclusive-factor underlying certain trends observed by demographers. In some cases, however, the influence of individual and social behaviour is decisive. In this article, Gilles PISON and Nadège COUVERT make use of long data series to provide a clear demonstration of this fact. The proportion of twin births has varied continuously since the eighteenth century at least, due to changes in the age of childbearing, wars, voluntary birth control or, more recently, the advent of techniques to overcome infertility. To mention just one of the remarkable results presented here by the authors, two-thirds of the increase in the twinning rate-from 0.9% to 1.5% of births-observed over the last thirty years can be attributed to medical infertility treatments.
The proportion of twin births - approximately one in 100 births in France until recently - was long considered to be a constant of the human species, depending only on biology, much like the proportion of male to female births, fixed at around 105 boys for 100 girls. Yet the rate of twin births has varied since the eighteenth century in France, rising at times and falling at others. And, since the beginning of the 1970s, the proportion of twin births has increased spectacularly, by almost 70%. How can these variations be explained? In this article, we show that a variety of factors, relating to both biology and behaviour, are involved. Because it is sensitive to effects where biology, medicine and society interact, the rate of twin births is a valuable indicator of biological and social change.
In the first part of this paper, we examine variations in the rate of twin births in France, particularly in the twentieth century, and we review the principal underlying factors. In the second and third parts, we examine in detail two factors which have thus far not been extensively studied: the voluntary limitation of births and selection through fecundity. Regarding the first factor, we study the extent to which a twin birth, an unanticipated event in the life of a family, modifies intended fertility. In particular, we seek to determine whether women who give birth to twins are less likely to undertake another pregnancy than women who give birth to a single child, while studying the consequences of such behaviour on the rate of twin births. In the third part, which addresses the issue of selection through fecundity, we focus on the First World War period during which the rate of twin births attained surprisingly high levels. We explain that this phenomenon can be attributed to a selection effect on more fecund couples. As demonstrated through the study of cohorts of newlyweds, hyperfecund couples have a higher twinning rate than their less fecund counterparts.
I. Variations in twinning rates in France since the eighteenth century: the role of "traditional" factors
In France, in the year 2000, 15 out of every 1,000 births-approximately 1 in 70 - was a twin birth (INSEE). This figure may well represent a new record, for the past two centuries at least. Figure 1 demonstrates this point, retracing the variation in twinning rates as far back as existing national data permit. Figure 2 provides more specifics, using the same data to show annual variations limited to the past hundred years only. Estimates of the twinning rate for the eighteenth and early nineteenth century are drawn from the Louis Henry historical survey of France(1) (Gutierrez and Houdaille, 1983). Though survey data cover rural communities only, they represented 85% of the total population at that time. The twinning rate was high, approaching 15 per 1,000 in the first half of the eighteenth century (Figure 1). It decreased in the second half of the century, falling to under 10 per 1,000 over the period 1790-1829. From 1858, annual estimates became available through registration statistics (Statistique de la France and INSEE; Daguet, 2002). …