General mechanisms
To facilitate the discussion of the rural-urban differences, some general causal mechanisms are first briefly reviewed, with special attention to the level at which they may play out in the Norwegian setting.
One reason why one might expect an effect of community education is that better-educated people may have learned about health at school; they may have become more conscious about the ability to influence their health; and their skills and credentials may have given them a higher income, in turn facilitating health-promoting activities [
43]. This knowledge, attitude and behaviour may be passed on to others through social interaction [
44]. It is not easy to define an appropriate level of aggregation for this mechanism. One typically interacts directly with a subgroup of people in the neighbourhood, who in turn interact with others. In addition, there is direct interaction with persons outside the immediate neighbourhood, and one observes other people's lifestyle in a more anonymous way through wide-covering media.
A second type of pathway that in principle may be relevant is that the higher average income resulting from higher education among people in the neighbourhood may contribute to a more pleasant physical environment, which may encourage outdoor physical activities and perhaps produce a general feeling of well-being. (In some countries, low crime rates in the richest neighbourhoods may add to the advantage.) Again, a broader area is also of relevance. One may benefit from the qualities of other areas not too far way, and if there are rich people in other parts of the municipality there will be higher incomes from taxation to spend on upgrading of all neighbourhoods in the municipality.
A third possible mechanism, operating in particular at the municipality level or even higher, is that the education of other people might affect the quality of the health services. As a background for this argument, some basic facts about the Norwegian health care system are necessary: The municipalities are responsible for primary health care, including for example health centres with general practitioners and nursing homes [
45]. These services are financed by the municipalities' tax revenues, various types of transfers from the national government (grants to the municipality, partly to compensate for low levels of local revenues, and reimbursements for each patient from the national social security system), and relatively small payments from the patients. In addition, many general practitioners operate on a private basis, but with much support from the municipality and the national government. Specialist health services are to an even larger extent public. During the period under study, the vast majority of the hospitals - each of which had responsibility for a certain group of municipalities - were owned by and financed by the government or the counties (the incomes of the latter stemming from taxation of their citizens and government grants). The additional private hospitals or individual specialists receive support from the government. Returning to the importance of socio-economic resources, it may possibly be easier to recruit qualified personnel to the various private and public health services in (or with special responsibility for) the municipality under consideration when many persons in that municipality or nearby are well educated. One might also expect that higher tax incomes in the municipality would contribute to a higher density of public health centres or nursing homes, or that higher purchasing power of the inhabitants might fuel the establishing of private health services, which also the socially less advantaged could benefit from. Unfortunately, there is little knowledge about these potential effects. In one study, a positive association between the economic resources available to the municipality government and the density of primary physicians was suggested, though the mean income level appeared to be unimportant [
46].
Fourth, a high level of education within an area that may be considered a local labour market may, of course, not only increase the income of "others", but also increase the chance that the person under consideration has a well-paid job and thus a high retirement pension later. A high income may in turn reduce mortality [
47] because of its implications for the person's health behaviour or (though less relevant in a society with a public health care system) his or her access to good health care.
A fifth possible mechanism, through probably less important, is the following: When other people have better health because of better education, and therefore present less competing demand for health services, the individual under consideration may receive better help. Given the organization of the health care system, the municipality level is especially relevant, but the competing demand in a broader area may also have some importance, and for nursing homes in the largest cities, the catchment area is often one or more boroughs.
Sixth, there may be an offsetting mechanism contributing to an
adverse effect of community education: If we compare among persons with the same level of education who live in different areas, those who live in neighbourhoods with a high average education have a lower education
relative to others in the neighbourhood than do those who live in neighbourhoods were people are not so well educated. It has been argued that a low relative
income may produce a psychosocial stress that increases mortality [
15,
48‐
51], and perhaps a low relative
education has a similar impact, although it is typically less visible. Just as for the learning argument, a relevant level of aggregation is difficult to define.
Possible reasons for differences between small and large municipalities?
The argument about pleasant physical environments is probably not very important in Norway, where even the poorest neighbourhoods do not look too bad, and green and perhaps quite unspoiled areas are never far away. It is of particularly little importance in the more rural areas. Below, it is discussed whether also the other causal pathways suggested above might be less powerful in the less populated (and more rural) municipalities.
Let us first consider the mechanisms involving social interaction or comparison with others, which are particularly likely to operate at a low level of aggregation. In the smallest municipalities, the BSUs tend to include fewer persons and cover a larger area than in the larger municipalities. Should we expect that the influence from other people in the same BSU, through learning, imitation or comparison, is weaker when there are fewer of these people and the distance to them is larger? That is far from obvious. For example, while it may be easier to meet people when distances are short, high population density may also strengthen the need for privacy. Empirical studies have provided mixed conclusions. Some have suggested that low population density reduces the amount of social interaction [
52], while others have pointed in the opposite direction [
53].
The other mechanisms probably operate largely at a higher level, i.e. also the socio-economic characteristics of neighbouring BSUs may affect mortality [
54‐
56] through these pathways. Unfortunately, there is no information about neighbouring BSUs in the data (see comment on BSU identifiers above), but it seems reasonable to base the discussion on an assumption that there is some clustering, in the sense that low-education BSUs in a municipality are more likely than the high-education BSUs to have low-education BSUs as neighbours.
Let us first assume that the degree of clustering is the same throughout the country. Certainly, the generally higher educational level in the large municipalities means that low-education BSUs in small municipalities are more likely to have low-education BSUs as neighbours than are the low-education BSUs in large municipalities. However, that is the case also for high-education BSUs. The assumption about homogenous clustering means that the difference between low- and high-education BSUs in the proportion low-education BSUs among their neighbours is the same for small and large municipalities. In such a situation, one possible reason for the pattern in the estimates may be the following: In the largest municipalities, the neighbouring BSUs within the area under consideration may have a large enough population to function as a local labour market or a catchment area for health institutions, making the arguments above about well-paid jobs and high-quality health care particularly relevant. In contrast, these factors may be influenced by BSUs farther away, which may be more different, in the smaller municipalities. (The social interaction mechanisms also involve this higher level of aggregation [
57], but as mentioned earlier, and with relevance also at a higher level, the interaction with population density is not obvious.)
The other possibility is that the degree of clustering actually does differ between small and large municipalities, so that the health of those who live in a low-education BSU in a large municipality is more negatively influenced by characteristics in the wider community than is the case for those living in high-education BSUs, while there is less difference between the neighbours of low- and high-education BSUs in rural areas. That would accord with the sharper relationship between individual mortality and BSU average education observed in the largest than in the smallest small municipalities. Unfortunately, the empirical underpinning for such an idea is weak. There appear to be some differences between the largest Norwegian cities in the degree of spatial segregation above the BSU level [
58], but the rural-urban differences have not been checked (and it could not be done with the data available here). Recent American studies based on measures of dissimilarity at different levels of aggregation have not documented any such relationship with population size either [
59,
60].
Confounders
In addition to the causal effects discussed so far, the estimates may reflect various selection mechanisms. One is that certain characteristics of the BSU or a larger area that increase people's chances of taking much education or that attract people with high education (e.g. physical environment, economic resources, or cultural values) also may affect mortality. In this study it is only controlled for the population size of the BSU and the municipality and the average age. In principle, the rural-urban differences in the estimates may reflect that there are other additional determinants of education in small than in large municipalities. For example, low-education BSUs in large cities may to a larger extent than low-education BSUs elsewhere be located near major traffic routes, which may increase mortality.
Further, the estimates may be partly a result of individual unobserved characteristics because of selective migration. More precisely, people who live in a BSU where the average education is high may be different (beyond what we can measure with the available variables) from those living in other BSUs, and not as a result of the high average education, which would simply be a causal pathway, but because some characteristics may increase the chance of moving to or remaining in a place with many better-educated. These characteristics may also affect mortality. For example, one might speculate whether high-class areas in large cities are particularly popular, perhaps because their advantages for some reason are more conspicuous, and therefore attract a special type of "successful" people who would have low mortality anyway.