- We only consider the adult population aged 25 and over. This excludes students who temporarily emigrate to complete their education. In addition, as it will appear in the next section, it will allow us to compare the numbers of migrants with data on educational attainment in source countries. It is worth noticing that we have no systematic information on the age of entry. It is therefore impossible to distinguish between immigrants who were educated at the time of their arrival and those who acquired education after they settled in the receiving country; for example, Mexican-born individuals who arrived in the US at age 5 or 10 and graduated from US high-education institutions are counted as highly-skilled immigrants. As mentionned above, Beine et al (2007a) provided corrected measures by age of entry and found a very high correlation with the uncorrected numbers.

- Migration is de...ned on the basis of the country of birth rather than citizenship. Whilst citizenship characterizes the foreign population, the "foreign-born" concept better captures the decision to emigrate<sup>8</sup> . Usually, the number of foreignborn is much higher than the number of foreign citizens (twice as large in countries such as Hungary, the Netherlands, and Sweden)<sup>9</sup> . Another reason is that the concept of country of birth is time invariant (contrary to citizenship which changes with naturalization) and independent of the changes in policies regarding naturalization<sup>10</sup> . The number of foreign-born can be obtained for a large majority of OECD countries although in a limited number of cases the national census only gives immigrants’citizenship (Germany, Hungary, Italy, Japan and Korea). It is worth noting that the concept of foreign born is not fully homogeneous across OECD countries. In most receiving countries, foreign born are individual born abroad with foreign citizenship at birth. In a couple of countries, foreign born means “overseas-born”, i.e. an individual simply born abroad.

- We distinguish three levels of education. Medium-skilled migrants are those with upper-secondary education completed. Low-skilled migrants are those with less than upper-secondary education, including those with lower-secondary and primary education or those who did not go to school. High-skilled migrants are those with post-secondary education<sup>11</sup> . This assumption is compatible with Barro and Lee’s human capital indicators (based on the 1976-ISCED classi...cation). Some migrants did not report their education level. As in DM06, we

> 8In some receiving countries such as Germany, immigrants’children (i.e. the second generation) usually keep their foreign citizenship.

> 9By contrast, in other OECD countries with a restricted access to nationality (such as Japan, Korea, and Switzerland), the foreign population is important (about 20 percent in Switzerland). 10The OECD statistics report that 14.4 million foreign born individuals were naturalized between 1991 and 2000. Countries with a particularly high number of acquisitions of citizenship are the US (5.6 million), Germany (2.2 million), Canada (1.6 million), and Australia and France (1.1 million).

> 11In the US case, this includes those with one year of college

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