Mendius and Hanson open by explaining the evolutionary basis for the brain's negativity bias — a tendency to register and remember negative experiences more strongly than positive ones.

Why this bias evolved and persists:

• Ancestral survival depended more on avoiding threats than on savoring positive experiences
• This asymmetry means negative experiences get encoded into memory more readily and durably than positive ones
• Understanding this built-in bias is presented as the necessary starting point for any deliberate effort to cultivate greater well-being

The authors frame subsequent chapters' techniques as necessary countermeasures to this default bias, rather than simply generic feel-good advice.