Satiety signals from stomach to brain take 15-20 minutes to register fully. Most people eat faster than this — finishing meals in 8-12 minutes. Slowing down lets satiety register, producing less eating with no willpower involved.
How to slow down
Put fork down between bites. Drink water with meals. Conversation slows eating naturally. Smaller plates create visual 'full plate' satiety. Setting timer for meal length initially helps.
Most fast eaters find they're full sooner and eat less when they slow down. The change usually shows within 2-3 weeks of practice.