For example, a 2014 empirical study showed that environmental policy tightening since 1990 has had no net negative effect on productivity growth in OECD countries. Less productive firms experience a temporary fall in growth, but the gains experienced by the most productive firms - by seizing new market opportunities and deploying new technologies - counter this effect. Another study examined the relationship between the stringency of environmental regulations and country exports of environmental goods, finding that stringent regulation positively affects countries’ specialisation in environmental products. Other analysis showed that market-based environmental instruments, such as taxes and trading schemes, tend to have a more robust positive effect on productivity growth than other measures.