Influence and manipulation are both attempts to alter the actions or propensity to act of another agent. The only difference between the two that can generally relied upon is that the one who calls it manipulation finds it distasteful or immoral. If you disagree I invite you to find a general principle cleanly dividing the following examples into manipulation and influence; all the better if it can be made uncontroversial.
A woman telling her long-term boyfriend that they're not getting married before he gets his doctorate.
A man teasing a friend.
A man teasing a female friend.
A man teasing a female friend, flirting.
A man teasing a female friend, flirting with intent.
A man teasing a woman he met ten minutes ago, flirting with intent.
A woman encouraging her son to become a teacher because the job security is good.
A woman encouraging her son to become a lawyer because he'll be better able to support her in her old age.
I'm not denying that manipulation and influence can be usefully distinguished. I do not believe they can but I haven't spent the last month thinking about it on and off. There might exist a Schelling point dividing bidirectional communication into socially acceptable influence and unacceptable manipulation, a lawyerly thing, useful but without any defensible reason to be there exactly but that it must be somewhere and we have come to an agreement that here will do. If you believe that there is a point that actually is pragmatically better than another by a real margin, please state it, and defend your proposition.
Agreed. But is there any point beyond expressing opprobrium in having two words if you're not really talking about anything except your own feelings? What I'm really asking is whether a behaviourist along the lines of B.F. Skinner would have been able to distinguish the two concepts. If not, it goes in the "may need to phrase this differently depending on audience but no actual difference in facts or anticipations" category.
It's been a long time since I logged into LW; I just saw this. Actually, I released a book this year in which I analyze manipulation fairly extensively through the lens of the pickup artist subculture. It's called Confessions of a Pickup Artist Chaser: http://clarissethorn.com/blog/2012/03/08/confessions-of-a-pickup-artist-chaser-now-available/