Neural Responses to Taxation and Voluntary Giving Reveal Motives for Charitable Donations

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Science  15 Jun 2007:
Vol. 316, Issue 5831, pp. 1622-1625
DOI: 10.1126/science.1140738

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  1. Fig. 1.

    (A) Study protocol. We scanned 19 females using functional magnetic resonance imaging (fMRI) while they were presented with transfers that affected their own account (starting amount, $100) and the account of a local charity. Half the transfers were mandatory, to resemble taxation; the other half were voluntary. We explained that the experimenters would not know their choices and that one mandatory and one voluntary transfer would be randomly chosen and implemented after the experiment. Events for each trial occurred as presented in the time line, details are in the supporting online material (16). After a 1-s fixation dot, the screen revealed whether this trial's transfer was mandatory or voluntary, as well as the dollar amount change to the accounts of the subject and the charity. After 9 s, two vertically aligned labels were added in the lower portion of the screen, specifying the vertically aligned buttons on a response box. For mandatory transfers, one of the labels read “acknowledge” and the other “invalid button.” For voluntary transfers, one of the labels read “accept” and the other “reject.” Label positions varied randomly from trial to trial. Immediately after the subject's response, a four-point satisfaction rating scale was shown, to which subjects responded by pressing one of four laterally oriented keys on the button box. The rating scale disappeared after 6 s, and there was a blank screen for an intertrial period that was randomly jittered between 6, 7, and 8 s. (B) Study design. The cells show the dollar transfers. Each design cell was implemented three times as a mandatory transfer and three times as a voluntary decision. Orange cells indicate pure gains to the subject; green cells indicate pure gains to the charity. These pure-gain design cells from the mandatory condition were used to predict voluntary giving in the purple cells, where there was a tradeoff between the subject and the charity (see Fig. 4, A and B).

  2. Fig. 2.

    (A) Subjects' choices during voluntary transfers as a function of payoffs to the subject and the charity. Many transfers that were costly to the subject but benefited the charity were accepted, and the rate of acceptance increased as the cost of making a given transfer declined. (B) Subjective satisfaction ratings as a function of payoffs to the subject and the charity, as well as the voluntary-mandatory factor. Subjective satisfaction increased as transfers increased and costs decreased and was higher in the voluntary (solid lines) than in the mandatory conditions (dashed lines).

  3. Fig. 3.

    Neural response in the ventral striatum to mandatory payoffs for the subject (yellow), the charity (blue), and both (green).

  4. Fig. 4.

    (A) Predicting giving from activations in mandatory “pure-gain” conditions. We created measures of neural activation in response to “pure subject gain” and “pure charity gain” by averaging activation from the mandatory conditions where the subject received money at no cost to the charity and where the charity received money at no cost to the subject (orange and green cells, respectively, in Fig. 1B). We used these two sets of activations as independent predictors of the average acceptance rate in the nine design cells involving a tradeoff (purple cells in Fig. 1B). The figure shows standardized probit regression coefficients from models including subject and charity stakes as control variables and neural response to pure subject gains and pure charity gains as independent predictors. The dashed lines indicate P = 0.05 significance. Higher response to pure subject gain was consistently associated with less giving. Higher response to pure charity gains was consistently associated with more giving. Coefficients for individual predictors were reliable in seven out of 12 cases. (B) Differences in activation predict giving. As an overall measure, we averaged the neural activation measures across all six brain areas and computed the difference between neural responses to the charity's pure gains and the neural responses to the subject's pure gains (orange and green cells in Fig. 2B). Giving increased as the neural response to pure charity gains outweighed the neural response to pure subject gains (R2 = 27%, P = 0.02).