Migration theories can be classified according to the level they focus on. Micro-level theories focus on individual migration decisions, whereas macro-level theories look at aggregate migration trends and explain these trends with macro-level explanations. The meso-level is in between the micro and macro level, e. g. on the household or community level and can explain both causes and perpetuation of migration.

Macro-theories of migration

The neoclassical macro migration theories explain migration as part of economic development. Internal migration occurs as a result of geographical differences in the supply and demand of labour, mostly between the rural traditional agricultural sector and the urban modern manufacturing sector, according to Hagen (2008). The basic model (Lewis, 1954 and Ranis & Fei, 1961) that grew out of trade theory, assumes perfect markets and a labour surplus in the traditional agricultural sector that is absorbed by the modern sector. The modern sector grows through capital accumulation and by poaching labour from the traditional sector. Rural workers are attracted by the positive wage differential and migrate to the urban sector, i. e. they are pulled to migrate. In these models migration occurs until wage equalisation has occurred. Todaro and Harris (Todaro, 1969 and Harris & Todaro, 1970) augmented this model to account for the significant urban unemployment that was found in many less developed countries. Migration is not completely risk-free, because the migrant does not necessarily get a job upon arrival in the city. Migration may increase if urban wages increase or the urban employment rate increases (ceteris paribus).

The authors (Todaro & Harris) show that it can be perfectly rational to migrate, despite urban unemployment, due to a positive expected income differential. This model has clear predictions and while the significance of income differentials is undisputable in labour migration decision, it is probably not as excessive as Harris and Todaro describe it. The model assumes that an equilibrium will take place, which we do not find in the real world and some of the other empirical predictions e. g. wage equalisation, have also not been found. The dual labour market theory (Priore, 1979) explains migration as the result of a temporary pull factor, namely strong structural labour demand in developed countries. According to this not purely economic approach, there is economic dualism on the labour market of developed countries and wages also reflect status and prestige. There is a primary sector providing well-paid jobs and a secondary sector, for unskilled jobs, e. g. manufacturing.

The demand for migrant labour force stems from several factors. This model is important because it explains some of the post-war migration trends in Europe and the United States, but the focus is too narrow with only one pull factor being analysed and with no deeper analysis of migrant decision making. Another macro-level model explaining rural-urban migration in less developed countries is Mabogunje’s (1970) migration as a system model, in which he explains migration as a dynamic spatial process. Aggregate migration flows and interactions are modelled by starting with a pool of rural potential migrants that is affected by various factors in the decision to migrate. The rural control sub-system controls outflows (e. g. family or community norms), the urban control sub-system controls inflows (e. g. through employment agencies), feedback is channelled back to potential migrants and the background environment also affects migration flows (social & economic conditions, government policies, transport & communications infrastructure etc. ). The environment and sub-systems are constantly changing, also as a result of the migration flows, which makes the system open and dynamic. Other authors (e. g. Kritz & Zlotnik, 1992) have also emphasised the importance of viewing international migration as an interdependent dynamic system, with own but interlinked systems for sending and receiving countries and feedback and adjustment coming from the migration process itself. It can also be linked to the world systems theory, discussed above. It is important to take note of interactions between different actors and to emphasize the dynamic nature of migration. Nevertheless the migration system models are vague and do not allow concrete prediction of migration trends.

Politics matter in migration theory, as migration laws and thus the right to cross a border legally directly influence migration flows. These laws of nation states are the result of the relative power of different interest groups. They are influenced by profits (thus linking up with Priore’s dual labour market theory), national identity, considerations of national security and the extent of multiculturalism in the state. In a historical analysis it is important to point out that acquired rights, laws or existing institutions will always influence migration flows, irrespective of economic considerations, like the business cycle (Hollifield, 2000). Globalisation not only affects the demand for labour or facilitates migrant networks, but also leads to loss of border control. Zolberg (1981) argues that it is not just the economic factors that matter in making the structural setting of migration, for example some of the countries that would be considered peripheral in the world systems theory (the Communist countries) chose to do so due to political reasons and political motives also influence migration flows (e. g. of refugees). The political setting is thus an important structural factor in migration decisions. Zelinsky’s hypothesis of mobility transition (1971) argues that migration is part of the economic and social changes inherent in the modernisation process. It is part of the wider range of functionalist theories of social change and development, which try to link theories to past empirical trends. He argues that patterns and rates of migration can be closely linked to the stage of modernisation (e. g. industrialisation) and demographic factors (e. g. high birth rates). He emphasizes that the preference for more personal freedom is part of the modernisations process. While his theories broadly make sense when looking at past migration patterns in industrialised nations, it is vague and does not allow differentiation of different types of migration and it does not consider the individual migration decisions.

Micro-theories of migration

Lee (1966) was the first to formulate migration in a push-pull framework on an individual level, looking at both the supply and demand side of migration. Positive and negative factors at the origin and destination push and pull migrants towards (non) migration, hindered by intervening factors, e. g. migration laws and affected by personal factors, e. g. how the migrant perceives the factors. He makes a number of predictions, for example that greater diversity among people leads to more migration and for this reason there are high rates of migration within the United States. This theory is barely a theory, it is more a grouping of factors affecting migration, without considering the exact causal mechanisms. The human capital approach is the neoclassical micro-level migration theory. Based on the work of Sjaastad (1962) migration is treated as an individual investment decision to increase the productivity of human capital, thus again focusing on the labour market, but at the same time explaining the selectivity of heterogeneous migrants. Individuals make a rational cost-benefit calculation of the expected discounted returns of migration over future time periods, migrating only when the expected returns are positive.

Wolpert’s stress-threshold model (1965) describes a behavioural model of internal migration, similar to a cost-benefit analysis, but assuming individuals that intend to be rational ex-ante, but are not necessarily so ex-post. Individuals have a threshold level of utility they aspire to. They compare place utilities to this threshold in order to decide whether to migrate or not and to which place. Place utilities for the current position are based on past and future rewards, whereas place utilities for possible destinations depend on anticipated rewards. Knowledge is based on the subjective and 11 incomplete knowledge that individuals have in their personal action spaces, so rationality is bounded. Action spaces depend on personal characteristics, the variability of the environment and life-stage of the individual. Migration flows thus occur as a consequence of these individual place utility evaluations and are not necessarily optimal according to rational and perfect information standards. This model leaves out some of the unrealistic assumptions of the human capital approach, but it is even more difficult to test. In a way Wolpert only changes the terminology, compared to the human capital approach. Another behavioural model, the value-expectancy model (Crawford, 1973) is a cognitive model in which migrants make a conscious decision to migrate based on more than economic considerations.

The potential migrant’s strength of migration intentions depends on a multiplication of the values of migration outcomes and expectations that migration will actually lead to these outcomes. Values are specific goals, e. g. wealth or autonomy. Values and expectations depend on personal and household characteristics (e. g. education level) and societal norms. These values do not necessarily need to be economic, for example security or self-fulfilment can also be important to potential migrants. Migration depends on the strength of migration intentions, indirect influences of individual and societal factors and modifying effects of constraints and facilitators. It is similar to the place-utility approach of Wolpert and again it shows that migration choices are subjectively made. There are also other similar micro-based individual behavioural decision making models, e. g. work by de Jong & Fawcett (1981) or the adjustment-to-stress approach of Ritchey (1976). The behavioural approach also considers non-economic factors and societal influences, but it is very vague and rational decision making is still assumed. Complimentary to the dual labour market theory is Hoffmann-Novotny’s approach of explaining migrations as a theory of social systems (Hoffmann-Novotny, 1981).

According to this theory migration is a result of resolving structural tensions (power questions) and anominal tensions (prestige questions). Migrants hope to achieve their desired status in the destination country, but often tensions are transformed instead of reduced. How successful they are depends on the global distributions of the different systems (for the different countries) among “status lines”. A migrant coming from a country with a low rank is unlikely to achieve a high internal rank at the destination. “Undercasting” of migrants takes place, which means that migrants take on the lowest position in society, whereas lower stratum natives experience upward mobility, at least in terms of power/ income. This theory does not exclude economic push factors for migration, but instead places them in a wider context of other societal push factors and also considers what happens to migrants at their destination. The theory broadly makes sense and furthermore includes structural factors, which most micro theories neglect.

Family-decision making and the New Economics of Labour Migration

The more traditional migration approaches focus either on aggregate migration movements or individuals making migration decisions. They thus assume that individuals independently make the decision to migrate. Some of the migration literature includes a seemingly wider decision-making framework, for example Harbison (1981) paper is entitled “Family Structure and Family Strategy in Migration Decision Making”. However, the migration decision is still not seen as a strategic family decision; the paper only acknowledges that families can influence the individual migrant’s decision, e. g. through the demographic structure. When looking at migration from a gender perspective, family structure can influence the migration decisions of women in particular. As Morokvasic (1984) points out, women migrate not only because of economic motives, but also to get married, due to social constraints, low rights and lack of protection against domestic violence. Sandell (1977) and Mincer (1978) on the other hand view migration as a family decision. The family as a whole migrates if their net gain is positive. If only one partner finds a (better) job at the destination, the family only migrates if gains of one family member internalise the losses of the other family member.

The family migration decision is thus in essence an aggregation of individual migration utilities. Bigsten (1988) also considers migration a household decision in which a family allocates labour to the urban or rural sector depending on the marginal products of combined wages. The New Economics of Labour Migration (NELM) goes one step further. According to this approach decisions are often made by household members together and for the wellbeing of the family as a whole1. Households also do not migrate together (as in Mincer, for example), but rather send of one or more household members off as migrants. The NELM is a fundamentally different theory of migration that was developed by Oded Stark in the 1980s, in cooperation with David Bloom, Eliakim Katz, David Levhari, Robert Lucas, Mark Rosenzweig, and J. Edward Taylor. The NELM is the only migration theory that explicitly links the migration decision to the impacts of migration, with remittances being this link (Taylor & Fletcher, 2001).

According to the NELM a household maximises joint income, status and minimises risks. All three aspects contribute to the migration decision of the household. Each of those will now be discussed in turn. Basically, what this paper states is that if income and or status are low and or risks are high and there are market and government failures (so no other means of social protection), the household then makes the decision that one or more members of the household migrates to a labour market that is negatively or non-correlated with the local labour market. They thus co-insure each other by diversifying their labour portfolio. Both the diversification of income and the investment-enabling character of remittances matter to the migration strategy. Households cooperate to achieve the income diversification and informal insurance and also benefit from intra-household specialisation (migrants/ non migrants).

Finally, the NELM should not be seen in isolation of the other migration theories. Stark includes the income differential approach to migration. Taylor (1986) points out that migration networks can be a source of information, thus increasing the certainty about returns in potential migration destinations. The NELM extends the migration decision to also include risk and insurance considerations, household decision making and links the migration causes to the consequences. Therefore it is a more realistic and useful theory, even though it is still quite abstract and stylised and has some strong assumptions, like rationality. Unlike the sustainable livelihoods literature it is also quite vague on the combination of different types of activities. Like most theories of migration it is also vague about how migrants choose where to go specifically.