Factors that can affect demand include: 1) Consumer income - Demand increases when consumer income rises through higher wages or lower prices for other goods. 2) Consumer tastes - Demand may change when products are successfully marketed or new trends emerge. 3) Substitutes - Demand for one product falls when substitutes become cheaper. Lower butter prices increase demand for margarine. 4) Complements - Related goods see demand rise together; lower DVD player prices boost DVD demand.