If you eat foods with hot peppers, you likely know the spiciness, or heat, can vary a lot.
The heat comes from a group of compounds called capsaicinoids, including the well known capsaicin. This fiery compound causes “chemesthesis” in which the receptors inside the mouth react to pain, touch, and heat. Some may call it pain, others call it pleasure.
Chile pepper varieties have a varying amount of heat and can also be quite different within the same variety. Growing conditions will also determine heat in peppers. If the plant is stressed, the peppers will produce more capsaicin.
The pith, or white membrane, contains a majority of the heat from capsaicin. Simply cut out the pith, as well as the seeds, to cool down the heat. Save these to add back to a recipe if more heat is desired. The size of pepper makes little difference in heat pungency.
Source: The Science of Good Cooking, Cook’s Illustrated