Higher incomes lead to more spending on imported items like French wine, foreign cars, and vacations abroad. Our exports are effected by the consumers in the other countries. Net exports tend to decline as income rises. Because the value of exports is influenced by consumers in other countries and the value of imports is effected by our income level, the total effect of the combination of the two on net exports is difficult to determine. For simplicity, economists say that the value of net exports is not related to level of income.
Net exports are autonomous to income.
When the value of the dollar increases relative to foreign currencies, foreign products are cheaper for Americans and U.S. products are more expensive to foreigners. This causes imports to increase and exports to decrease, resulting in a decrease in net exports. When the value of the dollar decreases relative to foreign currencies, exports increase and imports decrease. This causes an increase in net exports.