Genetic, linguistic, and geographic data can be used to explain the distribution of cultural units ("semes") and to understand the evolutionary mechanisms of culture. Three broad models of cultural transmission attempt to explain why cultures share semes: (1) Cultural diffusion, emphasizing horizontal transmission. (2) Local adaptation, where trail-and-error learning leads to the independent adoption of semes by different peoples living in similar environments. (3) Demic diffusion, which emphasizes vertical and frequency-dependent transmission. Authors test the explanatory power of each model using cultural, genetic, linguistic and geographic data from 36 African cultures.