In the Cobb-Douglas function Q = K L^a C^b, decreasing returns to scale result when:

1. In Q = K L^a C^b, the sum of exponents a + b governs returns to scale. 2. If $a + b > 1$ returns are increasing, if $a + b = 1$ they are constant, and if $a + b < 1$ they are decreasing. 3. Decreasing returns therefore require $a + b < 1$. 4. $a + b > 1$ gives increasing and $a + b = 1$ gives constant returns, while $a + b = 0$ is not a meaningful case here. _Source: ICAI BoS CA Foundation Paper 4 Business Economics, Ch 3 Unit I "Theory of Production", p.22_