The SRAS is upward sloping because input prices are sticky in the short run. This reflects the reality that rises in the price level boost a firm's profitability and offer incentives to expand output in the near run. Firm earnings decline when the price level falls, creating an incentive to reduce output. When aggregate demand rises, prices rise as well, but wages tend to be sticky, shifting slowly in reaction to price changes so that the marginal cost curve does not shift up with them. Firms find it profitable to increase production because price rises faster than marginal costs.
