Introduction to psychology by Paul Bloom (2007). This is an extraordinary course on psychology. Prof. Bloom uses an evolutionary point of view to understand the human mind. He explains very well complex subjects like religion or love.
One of my favorite parts is when Bloom uses the concept of parental investment to explain physical and behavioral differences in males and females. Females typically have a much higher parental investment than males: A female cannot mate anymore while it’s pregnant, but a single male could fertilize several females, forcing some males to go mate-less and giving rise to competition to see who can mate with the most females. This competition thus explains why males are typically larger than females. So for instance, elephant seals are four times larger–the males are enormous. They’re four times bigger than the females. And this is in large part because elephant seals compete for harems of females. It’s a “winner takes all.” For females, however, females can always find mates. Females don’t really have to approach males. Females, biologically, are choosy. And so males have to compete not merely with other males to get reproductive access but also to woo females. And so often, males have evolved special displays only to be attractive and to attract mates.