I just saw this abstract on SSRN — looks like a very interesting study that is, to me, not surprising, but a little depressing:

“Thanks for Nothing: Expressing Gratitude Invites Exploitation by Competitors”
Harvard PON Working Paper
JEREMY A. YIP, University of Pennsylvania – The Wharton School
Email: yip@wharton.upenn.edu
CINDY CHAN, University of Toronto – Marketing
Email: cindy.chan@utoronto.ca
ALISON WOOD BROOKS, HBS Negotiations, Organizations and Markets Unit

Previous research has revealed that expressing gratitude motivates prosocial behavior in cooperative relationships. However, expressing gratitude in competitive interactions may operate differently. Across five studies, we demonstrate that individuals interacting with grateful counterparts become more likely to engage in selfish behavior during competitive interactions. In Studies 1a and 1b, participants who interacted with counterparts expressing gratitude were more likely to make aggressive offers in distributive negotiations than those who interacted with counterparts expressing neutral emotion. In Study 2, we find that inferences of the tendency to forgive mediates the relationship between gratitude expression and selfish behavior. In Study 3, we contrast expressions of gratitude with another positive-valence emotion: excitement. We show that expressing gratitude promotes self-interested behavior compared to expressing excitement or neutral emotion. In Study 4, we find that gratitude expression triggers self-serving deception. Taken together, our findings suggest that expressing gratitude can be costly in competitive interactions: people infer that grateful counterparts are forgiving and, therefore, they are more likely to exploit their counterparts for selfish gain.