The inverse care law was suggested thirty years ago by Julian Tudor Hart in a paper for the Lancet, to describe a perverse relationship between the need for health care and its actual utilisation. In other words, those who most need medical care are least likely to receive it. Conversely, those with least need of health care tend to use health services more (and more effectively).
Inverse laws are of course commonplace, and essentially arise because of income inequalities. In most areas of life (politics of envy aside) most of us are reasonably happy with this state of affairs. The fact that the rich have more clothes than they strictly 'need' is not too great a cause for concern. But the fact that an inverse law applies to health care offends against most people's views about fairness - a view which forms the basis for the existence of the NHS.
The concern about fairness arises from a deeper view about the distribution of health. Inequalities in health arise, not only from variations in access to health services, but also variations in the quality of health care from area to area. And of course, variations in factors outside the control of the NHS - wealth, lifestyle, genetic and environmental considerations - will all affect people's health status.
There is considerable evidence that many populations, particularly those living in areas of high socio-economic deprivation, suffer on all three counts: they use poor quality services, to which they have relative difficulty securing access and they suffer multiple external disadvantage.
Although Tudor Hart did not provide hard evidence to support his hypothesis, many others have. There is also plenty of evidence now from routine health service data - such as the NHS performance indicators and from the surveys of NHS patients.
In areas with high needs, such as inner cities and deprived areas, there tend to be fewer doctors working with higher caseloads and sicker patients.
Although GPs are encouraged to work in 'underdoctored' areas through a system of incentives, these have not enticed enough GPs to work in the poorest areas.
Other evidence suggests that there are problems with the service some GPs are delivering in deprived areas. For example, the National Survey of NHS patients' attitudes to General Practice in 1999 showed that a significantly higher proportion of people living in deprived areas reported putting off a visit to see the GP because of inconvenient hours. Similarly, a significantly higher proportion of people living in deprived areas felt like making a complaint about staff - but had not actually done so.
Also, rates of immunisation, and screening for cervical and breast cancer, are significantly lower in people from more deprived areas - areas where cancer mortality rates are highest. The quality of treatment in general practice for people with chronic diseases such as asthma has been shown to be inadequate, with significantly higher admission rates to hospital for these conditions from deprived areas.
The NHS Plan aims to tackle avoidable health inequalities - partly through performance management to improve poor quality services and reduce variation in service provision. The existing NHS resource allocation formula (currently under review) links resources to identified need and the interim health inequalities adjustment is an attempt to target more resources to areas of high health inequality.
However, if the NHS is to tackle health inequalities from poor life chances it will need to target deprived areas in new ways - such as accelerated access to secondary care surgery.
(To go with graphs - to come) Index of local conditions This is a composite measure of deprivation compiled by the Department of the Environment and based on data from the 1991 Census. The variables used are: unemployed persons, children in earning households, overcrowded households, residents in households without basic amenities, households without a car, children living in unsuitable accommodation, and educational participation.
John Appleby is director of the health systems programme at the King's Fund.
Chris Deeming is research officer, health systems, at the King's Fund