Decreasing returns to scale — output grows slower than inputs: inputs ×2 → output ×1.866

An isoquant joins the labour–capital mixes that give the same output. As labour rises, the capital you can drop while holding output keeps shrinking — diminishing MRTS, which is why the curve bulges toward the origin. Scale every input up together: more than in proportion is increasing returns, equal is constant, less is decreasing.

Model assumption: Cobb-Douglas Q = A·L^a·K^b (elasticity of substitution 1). Convexity, diminishing MRTS and returns to scale are in the study guide; the functional form is a learning supplement. Curves never touching the axes is a property of this form only. Returns to scale is about all inputs together, not diminishing marginal product of one.