Trade agreements are not just about improving trade conditions, but also about governments making promises to local interest groups. When capital can move easily between different industries, and when governments are strongly influenced by politics, trade agreements tend to involve more significant reductions in trade barriers. These agreements may also include limits on how high tariffs can go. In a dynamic model, tariffs are lowered in two steps: first quickly, then gradually, with the speed of change depending on how easily capital can move around.