This work focuses on the impact of sharing contexts on consumers’ decision processes and purchase-amount decisions. Four studies, using both hypothetical and real (incentive-compatible) choices, find that people regularly purchase more in sharing (vs. non-sharing) contexts. Evidence is presented suggesting that a significant portion of this effect is driven by a cognitive bias arising in sharing contexts that focuses people on what they will give to others, and away from what they will receive from others. Consequences of this bias include the noted surplus in purchase amounts, over-consumption, and waste.