General levels of freedom and prosperity, more income, means more tourism.
+ Removal of visa restrictions, government investment, greater political freedom, increased stability of area (Vietnam after war in 60's/70's), single currency in Europe (Euro), weakening of currency in tourist destinations
- Political instability (i.e. Libya), introduction of visas, tourist tax, exit tax, closing of borders, weakening of domestic currency (intl. tourism more expensive), economic recession, cost of travelling