Stage 3: Attachment

Attachment is the long-term commitment to be with someone that provides feelings of security, calm and a reduction in the stress hormone cortisol. Another term for this phase, from Robert Sternberg's theories of love, is companionate love.

There are a few reasons why people are monogamous in the first place:

• If the male is present, he can protect the child from predators.
• It's easier to be with a mate than to constantly fight for a mate.
• Females needed more space to find more food, which forced males to spread out as well (spacing effect theory)