# Why are demand and supply curves shown as concave up?

Why is it that demand and supply curves are typically shown in graphs, including in Econ 101, as concave up (i.e., convex)?

I know what it entails mathematically—that if $$p(q_d)$$ is a demand curve, $$p''(q_d)$$ is positive, and if $$p(q_s)$$ is a supply curve, $$p''(q_s)$$ is positive, but what’s the intuition for why that’s usually the case?

• They needn't be. Those are just arbitrarily drawn curves. They are just as often straight lines. – user18 Jan 4 '20 at 4:11

Intuitively you are only willing to provide good Q if the marginal costs are less or equal to price $$MC \leq P$$. Providing goods to the market at price below marginal cost is irrational and hence inconsistent with basic assumptions behind supply and demand.