This is the first study we know of that directly examines the effects of pre-and post- payment vs. no payment in physicians, and the first to do so amongst neurologists. The results suggest that there is markedly differential effectiveness of social-psychological influences in this group: 'greed' doesn't work, but 'guilt' does.
Pre-payment incentives derive their effectiveness from 'guilt', or what social psychologists refer to as 'the norm of reciprocity' [8]. When the unsolicited book-token is received, there is no further (monetary) benefit to replying [9]; but there is a social-psychological norm that the recipient should respond: one kindness deserves another. In this condition, being sent a reminder letter would serve to reinforce the 'guilt' that the neurologist may be expected to feel by not having responded despite having kept the book token: this may account for the continued increased response in the pre-payment group to the second mailing, even though there was no incentive included in that mailing.
Post-payments, by contrast, impose little social obligation on the subject, but demonstrate a respect for the subject's time that may in itself be considered a favour to be returned, and - if large enough - may induce a desire on the subject's part for the promised reward [10]. Ten pounds sterling is not a great deal (about enough to buy one new paperback book), but it compares favourably with the rate of pay a consultant might receive from the UK National Health Service for the approximately ten minutes the survey should have taken to fill in (though considerably less than they might expect to receive from additional, private work). It may well be that the neurologists would have responded more enthusiastically to a larger post-payment, but at this level of incentive the neurologists showed no evidence that they were influenced by 'greed'. Indeed, of those sent the book token in advance, three returned it (one who had completed the survey and two who had not), and of those promised the book token, two wrote on the survey that they did not want it and a third that it be sent to charity. Unsurprisingly, there was no increased response to the second round amongst those promised the token: those who had been unmoved by the offer the first time round could not expect to be inspired by whatever vague recollection they might have of the offer some weeks later. It may seem that the effects of 'greed', while small, were only non-significant because of the sample size. While that may possibly be true, the sample for that incentive arose as a result of random allocation over a complete sample of the neurologists in the UK, so could not have been expanded.
How broadly these findings may be generalised is uncertain. Response to incentives has been shown to vary with specialty [5], and it may be that neurologists are particularly subject to 'guilt', or particularly immune to 'greed'. It may also be that these findings apply particularly to the UK, where doctors are well paid by a public health system, and where neurologists may be a particularly 'bookish' group. 'Greed' may be hypothesized to be less of an incentive under those conditions, whereas 'guilt' operates consistently across cultures [11].
This survey did not use the full array of features included in the 'Tailored Design Method' [12] or the other techniques shown to increase response rates [13], and it may be that a fully optimised survey would have found different results. Nevertheless, it did use some features, such as first-class stamps, multiple contacts, university sponsorship, and the incentive itself, of course, consistent with what is known about influencing physicians specifically [14].
Finally, any correlations amongst the neurologists' responses in each cluster would mean that we had underestimated the standard errors and would increase our confidence interval for the odds ratio; though the intra-class correlation coefficient of responses was low (0.07), some chance of type 1 error therefore remains since we did not account for clustering in the analysis.