What happened
Mixed outcomes. The Swedish pooled differences were not statistically significant; the authors reported crowding out among women and reversal with the charity option. US evidence found increased donations at incentivized drives, partly offset by movement from neighboring drives. Context and the measured outcome change the conclusion.
- Mellström and Johannesson report pooled participation of 43%, 33%, and 44% across treatments, with differences not statistically significant; the women’s subgroup showed a significant payment decline. Crowding Out in Blood Donation: Was Titmuss Right?
- Their intervention concerned agreeing to become a donor, not a demonstrated long-term change in safe blood supply. Crowding Out in Blood Donation: Was Titmuss Right?
- In US blood-drive data and a field experiment, incentives increased donations at participating drives, but some of that gain came from donors switching away from neighboring drives. Will There Be Blood? Incentives and Displacement Effects in Pro-social Behavior
What might explain it
Plausible
A payment may alter the social meaning of an altruistic action differently for different participants. Crowding Out in Blood Donation: Was Titmuss Right?
Alternative explanation: Small subgroups, pre-existing preferences, or context-specific interpretations could explain heterogeneity without implying a stable gender rule.
Supported in case
A local participation increase can overstate a system-wide increase when people switch locations. Will There Be Blood? Incentives and Displacement Effects in Pro-social Behavior
Alternative explanation: Some additional attendance may represent genuinely new or better-timed donations; displacement need not eliminate all benefit.
Understanding the outcome
Motivational crowding and displacement
Separates reduced willingness in one setting from substitution and increased observed donations elsewhere. Crowding Out in Blood Donation: Was Titmuss Right? Will There Be Blood? Incentives and Displacement Effects in Pro-social Behavior
Cash versus other incentives, prospective versus established donors, and outcome definitions prevent treating these as identical interventions.
Questions to take into practice
- Distinguish willingness, attendance, completed donation, and total supply before comparing studies.
- Track neighboring venues and later participation when evaluating a local incentive.
What remains uncertain
- Agreement to become a donor, completed donations and sustained safe blood supply are different outcomes. Neither a small subgroup’s response nor an increase at selected drives settles the long-term effect of a payment policy on the whole supply system.