Introduction to psychology by Paul Bloom (2007). This is an outstanding course on psychology. Professor Bloom uses an evolutionary perspective to understand the human mind. He explains complex topics like religion and love very clearly.
One of my favorite parts is when Bloom uses the idea of parental investment to explain physical and behavioral differences between males and females. Females usually have a much higher parental investment than males. A female cannot mate while she is pregnant, but a single male could fertilize several females. This leaves some males without mates and creates competition to see who can mate with the most females. This competition helps explain why males are often larger than females. For example, male elephant seals are four times bigger than females. Much of this size difference comes from the fact that males compete for harems of females. It is a “winner takes all” system.
For females, however, finding a mate is not as difficult. Biologically, females are choosy. Males must compete not only with other males for reproductive access but also to attract females. As a result, many males have evolved special displays to be more appealing to potential mates.