The Frequency of Twin Births in France: The Triple Influence of Biology, Medicine and Family Behaviour

By Pison, Gilles; Couvert, Nadège | Population, November/December 2004 | Go to article overview

The Frequency of Twin Births in France: The Triple Influence of Biology, Medicine and Family Behaviour


Pison, Gilles, Couvert, Nadège, Population


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). They show that the twinning rate stood at around 10 per 1,000 in the second half of the nineteenth century. It then increased in the 1890s, and the upward trend continued into the first decade of the twentieth century. The First Word War saw a sudden and short-lived rise (Figure 2)(2), though the twinning rate returned to just under 11 per 1,000 after the war, fluctuating annually between 10 and 11 per 1,000, and remained at this level until the early 1960s. A slight downward trend was observed from the end of World War I to the end of World War II, followed by a small upward trend in the following 20 years. In the early 1960s, the proportion of twin births fell, reaching a low point of 8.9 per 1,000 in 1972. It then rose again quickly and almost continuously until the late 1990s before levelling off at around 15.0 per 1,000 in recent years. …

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