The problem with the emerging concept of social capital is that it is in danger of trying to be all things to all people. Dr Walkup is correct to point to the view of Portes and others that social capital can be individual. I do not think that this approach is particularly useful. Social capital is not a thing, it is a way of trying to describe a number of social processes. It is a theory that helps us understand what is happening in a society. Although there may be analogous processes occurring at group and individual levels, conceptualising them as the same thing is problematic.

Theories of causation argue that causes at different levels are often governed by different rules and need different methods of investigation. An example would be the effects of smoking on health. This can be investigated at a number of levels; there would be the cellular level (the effects of nicotine on the cell), the individual level (physical and psychological effects of smoking and addiction) and the group level (what increases smoking levels in one group compared with another).

One would not try to employ the concept of cellular biology to investigate groups of people and one would not try to use group or systems approaches to investigate the individual. Moreover, the factors that increase the level of smoking in a group may not be the same as those that increase an individual's risk of smoking-related disease.

Given that group social processes are likely to affect health in different ways from individual processes, it would not seem helpful to consider social capital as a single entity that works at both levels. A choice has to be made and the choice of the majority is to conceive of social capital as operating at an ecological or group level and to consider effects at an individual level as social networks.

Dr Walkup is correct to point to the differences between the social relationships that allow a person to call on resources, and the resources themselves. However, the theory of social capital as an ecological variable does allow for this. Bonding and bridging social capital describe factors at the community level, but the concept of vertical social capital attempts to describe the ability of a community to facilitate access to resources from those in power.

Clearly, in our individualised world our interventions tend towards helping people decrease their risk of illness and their risk of relapse, and improve their participation in the world. The exciting difference about ecological conceptualisations is that they are about how society decreases the risk of illness and relapse of its population and how society facilitates the participation of the individual. These approaches aim for the same outcome but they are not the same thing and will need different conceptualisations, investigations and interventions.