And then what? The question most decision-makers skip.
First-order thinking sees the direct effect: a bounty on rats reduces rats. Second-order thinking asks "And then?": in 1902 the colonial administration in Hanoi paid per rat tail handed in. Soon tailless rats were running through the city, and people in the countryside were breeding rats for the reward. Every action creates reactions that change the action itself.
Useful: Before any decision, ask "And what happens then?" three times in a row.
Source: en.wikipedia.org