imports and domestic supplies. Their effects on trade will derive from the relative abilities of different suppliers to comply with these measures, the varied incidence of compliance costs and how each affects the relative competitiveness of different suppliers.

While there is general agreement that food safety and agricultural health measures do indeed strongly impact international agro-food trade, there is no consensus on the relative importance of individual measures and in relation to other trade-distorting measures, nor their aggregate net effect. Indeed, enormous difficulties are involved in empirical testing the impact of such standards on trade. Firstly, one must make assumptions about how the broad array of measures is actually enforced and how this deters or encourages potential export suppliers. Depending upon the enforcement regime, the adjustments needed by different suppliers may be very significant or quite modest. This variable cannot be aggregated and differs from country to country and among different industries. Secondly, there may be all kinds of secondary effects as a result of food safety and agricultural health requirements, for example leading to shifts in sourcing, affecting complementary and competitive goods, the spread of regulations and restrictions to other countries etc. Thirdly, often a specific measure is not a dominant or even very important determinant of observed trade flows, or lack thereof. There is a risk to ascribing shifts in trade to agro-food standards which are more fundamentally driven by other economic or technical factors. Fourthly, there are sometimes problems in defining the counterfactual situation against which the impact of such standards can be assessed. What would have happened in the absence of the measure? Would trade have been unimpeded or would distributors and consumers have sought product from other suppliers instead? In the absence of a (traderestricting) measure, might overall demand have declined for a product for which certain problems were identified? Finally, many food safety and agricultural health measures will affect domestic suppliers as well, with varied outcomes in terms of shifts in the relative competitiveness and market share of the different players.

These and other empirical problems have led researchers to devote more attention to specific cases and to attempt to highlight the role played by (changing) food safety and agricultural health requirements on bilateral or broader multi-country patterns of trade. A discussion of some such cases will be provided below. However, only one study has attempted to provide some kind of aggregated measure of the level of agricultural and food trade constrained or blocked by technical barriers. This was conducted in 1996 by the US Department of Agriculture (USDA). Drawing upon expert opinions by USDA and other regulatory personnel, it was found that ‘questionable’ technical barriers (that is measures judged to have no scientific basis) were inhibiting US exports of agricultural and food products to some 62 countries. More than 300 market restrictions were identified which constrained or blocked exports valued upwards of $5 billion, equal to around seven percent of US agricultural, food and forestry trade in 1996. Twothirds of the identified measures, including nearly all full or partial import bans, addressed risks for animal or plant health (Roberts _et al_ . 1999).

This type of broad estimate for the trade effects of food safety and agricultural health measures has not been repeated for any other country. However, partial insights into the subject have been obtained through other approaches. Most commonly, researchers have looked to the only available multi-country sources of ‘data’ on the subject, namely official listings of agricultural and food product detentions/ rejections by certain industrialized countries and the growing 17