Many authors consider it obvious that reciprocity supports PTA.
Furthermore, because not all participation carries substantial risk or burden, reciprocity supports social or researchers' obligations to participants less frequently than its proponents have thought.
It seems implausible that reciprocity should apply so rarely, particularly when what would make the difference as to whether reciprocity supports benefits is facts about supply and demand of participants.
(2) Reciprocity supports PTA for participants seldom: at most when participants have borne a significant net risk or burden, PTA to the trial intervention is necessary to keep them in sufficient health to enjoy the benefits owed to them, and there are no defeating reasons.
However, recipients of support were not more likely to reciprocate support in conflicts involving the supporter up to a week later, suggesting that reciprocity of support in ravens is not based on short-term calculations on a tit-for-tat basis.
Thus, reciprocity of support, or interchange between support and preening, can be examined while controlling for symmetrical features of the relationship such as kinship and sex.
Even when the Reciprocity Principle supports reciprocal obligations, competing principles can still override it.
An implication is that, assuming that few participants have purely altruistic motives for participation, the Reciprocity Principle supports reciprocal benefits for few participants.
The Reciprocity Principle supports reciprocal benefits for some participants, but not for the participant who assumes no risk but does assume a large burden for society's sake, such as enrolling in a long trial involving some discomfort.
