Endogeneity of fertility norm. It could be argued that the correlation between home fertility and fertility at destination is driven by reverse causality or unobserved heterogeneity. In particular, if migration ‡ows tend to respond to variables directly related to fertility rates, the previous results might be a¤ected by some endogeneity bias. For instance, if migrants choose their destination because of similarity in fertility behavior between the origin and the destination country, our key explanatory variable ln(n<sup>~~d~~</sup> ) = ln(<sup>P</sup> d<sup>�dnd) might be considered endogenous with respect to the fertility rate</sup> of the origin country through the endogeneity of �d; i.e. the proportion of migrants living in country d. Spilimbergo (2009) faces a similar problem when analyzing the relationship between democracy and foreign education since this latter variable is built using bilateral migration ‡ows of students. If foreign students were choosing the location of their education abroad on the basis of the democratic distance between origin and destination, then the relevance of the OLS results would be questioned.

Of course, it is neither obvious that migration decisions are made on the basis of similarity in fertility behaviors between origin and destination countries, nor that there is a missing variable such as cultural similarity related to fertility in‡uencing the decision to migrate. Focusing on South-North migration, one sees that a lot of developing countries with colonial links with a Northern country exhibit very high fertility rates and thus very di¤erent fertility behaviors between origin and destination countries. Colonial links are known to favour international migration (Mayda, 2005 for instance). Countries such as Congo, Chad and Mali exhibit the highest fertility rates (well above 6) while their colonizer (Belgium and France) is subject to low demographic growth. Nevertheless, since our migration data also include SouthSouth migration, one cannot exclude that there might be some degree of endogeneity of the migration rates with respect to fertility.

Given the fact that the possible endogeneity of ln(n<sup>~~d~~</sup> ) is related to the endogeneity of �d; there is no standard econometric procedure such as instrumental variable to deal with this issue. One intuitive econometric strategy is to use the predicted values of �d using a gravity model relating bilateral migration stocks to exogenous variables or variables not related to fertility. A similar procedure was used by Spilimbergo (2009) who uses predicted stocks of foreign students. The econometric procedure therefore involves two steps. In the ...rst step, we predict the migration weights on the basis of a set of bilateral exogenous variables. In turn, these predicted weights allow us to build an alternative measure of the fertility rate at destination. In the second step, we run the previous regressions using this alternative measure instead of the observed fertility rate at destination.

We start from the bilateral stocks of migrants from each origin country i to destination country d observed in 2000, Mid. We regress ln(Mid) on a set of three types of variables<sup>11</sup> : geographic distance in kilometers (DISid); existence of a colonial relationship after 1945 (COLid); since we also investigate South-South migration, a dummy

> 11The data come from the CEPII database dist_cepii including a large set of bilateral distance measures.

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