According to United Nation’s projections, by 2015 there will be 21 “megacities” of at least 10 million people —all but 4 in developing countries. In 1975 only 27% of people in the developing world lived in urban areas. In 2000 the proportion was 40%, and projections suggest that by 2030 the developing world will be 56% urban [46].
This growth in urban areas is a phenomenon that has been strongly influenced by migration. As an example of its magnitude, to date, approximately 10% of China’s population are rural-to-urban migrants [47].
Although the developed world is already far more urban, at an estimated 75% in 2000, urban areas of developing countries are growing much faster [48], and their populations are larger [49]. Hence, urbanisation —heavily influenced by migration—
poses a considerable challenge for public health, especially in developing countries [50-53]. Beyond these facts, it becomes a challenge to understand how city living is linked with a complexity of factors that have an effect on health [53, 54].
In Last’s “Dictionary of Epidemiology”, migrant studies have been defined as
“studies taking advantage of migration to one country by those from other countries with different physical and biological environments, cultural background and/or genetic makeup, and different morbidity and mortality experience. Comparisons are made between the mortality or morbidity experience of the migrant groups with that of their current country of residence and/or their country of origin. Sometimes the experiences of a number of different groups who have migrated to the same country have been compared” [55]. This definition does not consider the context and the impact of internal within-country, mostly rural-to-urban, migration.
Barry Bogin, in his “Patterns of Human Growth”, argues that migration redistributes the genetic, physiological, morphological, and sociocultural differences found in human populations [56]. In his book, a number of authors are presented as the first to publish studies on the growth patterns of urban migrants, dating from the late 19th to early 20th century. One of them, a paper by Boas titled ‘Changes in the bodily form of descendants of immigrants’ (published in 1912 and cited by Bogin) [57]
established that changes in growth were due to biological plasticity in the face of the new urban environment. This explanation refuted the idea that natural selection or a genetic mechanism could adequately account for the changes in growth [56].
The picture is complex, since the effect of migration on a particular outcome varies according to who is migrating, when they migrate, where they migrate from, where they migrate to, and what health outcome is measured [2]. Migration is further complicated by the fact that it is not necessarily a random process; the “selection of migrants” and the “healthy migrant effect” —or, in some circumstances, the unhealthy migrant effect— may influence health and disease risk [1, 2].
Such concern with selective migration is not new and this discussion topic dates back to 1938 [58]. Dorothy Thomas reviewed some studies evaluating the conflicting results of rural to urban migration studies which provided “apparently conflicting hypotheses as to the direction of this selection…: (1) cityward migrants are selected from the superior elements of the parent population; (2) cityward migrants are selected from the inferior elements; (3) cityward migrants are selected from the extremes, i.e., both the superior and the inferior elements; and (4) cityward migrants represent a random selection of the parent population” [58].
Thomas’ review discusses Sir Austin Bradford Hill’s 1925 study of migration and mortality in Essex [59] (referred to and cited by Richard Doll [60, 61] and Thomas [58]), which proceeded on the assumption that migration to the cities is selective of young adults. Bradford Hill observed lower age- and sex-specific mortality rates in cities when compared to rural areas [59-61]. “The observed differential for the age-selected groups favoured urban areas, in general, and especially females in urban areas, thus leading to the inference that at least part of this differential could be attributed to selective migration, and that therefore migrants to the cities represented, on the average, better physical risks than the residual population in rural areas” [58].
The observations made in the earlier in the 20th century [58, 59] are still relevant to most of today’s LMIC, since no single explanation was made owing to the complexity of the problem. Migrants to urban slums in today’s less developed countries do not necessarily experience the benefits of the urban environment. In
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Asia, Africa and Latin America the slums are often on the outskirts of the cities [56].
Not surprisingly, the growth of migrant children living in these slums is not significantly different from that of children living in the impoverished rural areas [56]. Migration due to economic reasons poses additional difficulties in interpreting studies, since those with better health or socio-economic status could be the ones more likely to “afford” to migrate. It could, therefore, be argued that migrant groups are self-selected groups.
Other related factors that complicate this panorama lie in the interpretation —as well as its applicability and generalisability— of research results derived from the available literature. For example, several studies show a lack of reference groups with similar characteristics that facilitate the comparison of the effect to be studied such as, people from the same place of origin who did not migrate. In other instances, difficulties are present with the selection of comparison groups that are not necessarily “similar” to each other, such as those studies that involved migrant groups from different generations.
Razum [62] outlined what would be the requirements of an “ideal” migrant cohort which includes a unique definition of “migrant” that considers duration of stay.
Additionally, and ideally, participants would have to be enrolled before they migrate, studies should include the population of origin of immigrants and studies should be based on individual data collected over time to understand the determinants of the relation between migration and health [62].
Due to the complexity outlined in this section, it is, therefore, important to be aware of these issues due to its impact in the design of new research studies, as well as in the interpretation of their findings.