Why black men have a bigger penis. The men in Scandinavia have on average a smaller penis than people from Africa. We assume for the sake of argument that penis length is genetically determined although no scientific studies have identified the genetic regulators of penis length. Why would there be a size difference. Scandinavian women do appreciate the longer penis. The theory that there has been sexual selection towards a smaller penis in Scandinavian countries therefore becomes implausible. Evolution often works in strange ways. Here we propose a different theory on the size difference of the human penis. The penis is a well vascularized organ. In this way it differs to a degree from other primate penises. The penis is not only well vascularized the blood flow to the penis can also be regulated. This can only mean one thing. The penis is a heat exchanger! A comparable organ is the ear of the African elephant. It is well vascularized and has a large surface area. It is used to dissipate heat. The male penis is not only used for reproductive means, it is also a heat exchanger in the hot climate of Africa where humans emerged. A dual function of organs is not uncommon in the animal kingdom. The ears of the elephant are still used to hear. When humans moved to Europe they finally ended up in cold Scandinavia. A large penis was evolutionary detrimental to survival. It dissipated too much heat. It shrank to reduce the surface area. And that my friends is the truth and nothing but the truth so help me Darwin.