1.   A falling yield curve is explained by investors expecting short-term rates to be lower in the future.

2.   A humped yield curve is explained by investors expecting short-term interest rates to rise and long-term rates to fall.

3.   A rising yield curve can be explained by liquidity preference theory.

4.   A humped yield curve is explained by the combination of a descending yield curve plus an upward-sloping liquidity preference curve.