Is Tipping Voluntary? How about Bribery?

An interesting question:  If a public official solicits a bribe to do his job, that seems like extortion.

But suppose I offer a "tip" to the public official, to process my application faster than the others, because I need it right away.  The public official should say "no" to this tip, obviously.  But I would be better off if the official would accept the tip, because my loss from the delay is much larger than size of the tip.  That's why I offered it.

So, are tips voluntary?  Are bribes?  Can they be euvolountary?  Interesting article...

Here Is a Tip: Prosocial Gratuities Are Linked to Corruption

Magnus Thor Torfason, Francis Flynn & Daniella Kupor
Social Psychological and Personality Science, forthcoming

Abstract:
We investigated the link between tipping, an altruistic act, and bribery, an immoral act. We found a positive relationship between these two seemingly unrelated behaviors, using archival cross-national data for 32 countries, and controlling for per capita gross domestic product, income inequality, and other factors. Countries that had higher rates of tipping behavior tended to have higher rates of corruption. We suggest that this surprising association may be accounted for by temporal focus — people may tip and bribe others in order to receive special services in the future. Indeed, in a pair of follow-up survey studies, we find evidence that the link between tipping and bribery can be partly accounted for by prospective orientation.

So, okay.  Suppose this is right.  Are places where tipping is expected more corrupt?  Or do corrupt societies see tipping as acceptable?  And "receive special services in the future" sounds like a repeated game problem to me.  Raising the old question:  are you more likely to leave a large tip if you expect to go back to that restaurant?  And, is tipping voluntary?  Some economics of tipping.