Affluence relates to the average consumption of each person in the population. A common proxy for measuring consumption is through GDP per capita. While GDP per capita measures production, it is often assumed that consumption increases when production increases. GDP per capita has been growing steadily over the last few centuries and according to the formula I = PAT, called the impact equation, is driving up human impacts on the environment. The equation I = PAT was proposed and developed by Ehrlich, Holdren and Commoner in the early 1970s (Ehrlich and Holdren, 1971; Commoner, 1972). It recognises that the impact of a human population on the environment can be thought of as the product of the population’s size (P), its affluence (A), and the environmental damage inflicted by the technologies used to supply each unit of consumption (T). Sometimes, because of the difficulty in estimating A and T, per capita energy use is employed as a surrogate for their product. Some equate T with impact per unit of economic activity (Dietz and Rosa, 1994), and for others T is a rather fuzzy category covering all sources of variation apart from population and affluence (Fischer-Kowalski and Amann, 2001).
Alternatives to I = PAT
While the I = PAT equation quickly became established as the norm and has been used and cited by many organisations and individual people ever since, recently, various alternative formulations of the equation have been proposed.
Dietz and Rosa (1994) gave a stochastic (probabilistic) reformulation of the impact equation (STIRPAT – Stochastic Impacts by Regression on Population, Affluence and Technology) which they claimed facilitates the application of social research statistical tools to studies on I = PAT. Schulze (2002) proposed modifying the formula to I = PBAT, which calls attention ‘to the many behavioural choices that are immediately available to all individuals’. Schulz points out that affluence and technology do not dictate behavioural decisions. He gives the example of a person who is wealthy and only uses the most efficient devices, and whose environmental impact will still depend on whether or not the person is a profligate consumer.
Willey (2000) noted that consumption is influenced by lifestyle and organisation. Improved organisation in rich countries could lead to a reduced per capita consumption, but in poor countries, better organisation might lead to a huge increase in consumption. So he proposed changing the impact equation to I = PLOT (population, lifestyle, organisation, technology).
Fischer-Kowalski and Amann (2001) argue that the full understanding of the impact equation must take into account the variety of socio-economic systems in different countries and the effects of globalisation and trade.
All socio-economic systems for which the I = PAT question may be posed are embedded not only in natural environments but also in networks of social systems with which they interact. The very nature of this interaction seems to be of crucial importance for their environmental (and of course also their economic) performance, and this is even more so in the face of globalisation.
Commoner, B. (1972) The Closing Circle: Nature, Man, and Technology, London: Jonathan Cape.
Dietz, T. and Rosa, E.A. (1994) ‘Rethinking the environmental impacts of Population, Afﬂuence and Technology’, Human Ecology Review, 1(2): 277–300.
Ehrlich, P.R. and Holdren, J.P. (1971) ‘Impact of population growth’, Science, 171: 1212– 217.
Fischer-Kowalski, M. and Amann, C. (2001) ‘Beyond IPAT and Kuznets curves: globalisation as a vital factor in analysing the environmental impact of socio-economic metabolism’, Population and Environment, 23(1): 7–47.
Schulze, P. C. (2002). I=PAT, Ecological Economics 40; 149-150.
Willey, D. (2000) Some Hopes and Thoughts for the Future, Manchester: Optimum Population Trust.
For further reading:
Gaia Watch of the UK: I=PAT. An Introduction [Online]: URL: http://www.populationgrowth-migration.info/essays/IPAT.html [Last modified August 26, 2009; Retrieved: October 15, 2009].
Gaia Watch of the UK: Population and Growth Migration
See also Environmental Kuznets Curve.
This glossary entry is based on contributions by Willi Haas, Simron Jit Singh and Annabella Musel
EJOLT glossary editors: Hali Healy, Sylvia Lorek and Beatriz Rodríguez-Labajos