Two Stanford researchers set out to discover which is more persuasive: “I think” or “I feel.” For example, would it have a bigger impact on you if I were to say, “I think this is what you should do?” or “I feel this is what you should do?”
They conducted the study by writing two almost identical pieces on the merits of blood donation. One started off with, “My feelings about blood donation: I feel that donating blood is one of the most important contributions I can make to society...” etc. It used the word “feel” or “feelings” several times. The other message was identical, except the words “my thoughts” and “I think” were substituted for the words “my feelings” and “I feel.”
So which message was more persuasive? Turns out, it depends on the subject. If the person receiving the message tends to be more emotionally driven, then they will be more swayed by the word feel. If a person is more cognitively driven, then they will be more persuaded by the word think.
So if you know which way the person you're speaking to leans, you know which word to use.
But the researchers didn't stop there. They also tested the differences in gender and found that men are typically more persuaded by “I think” and women are more persuaded by “I feel.” These are generalizations, of course, and there will always be exceptions.
Bottom line: If you know that your audience is predominately a “feeling” type of group or mostly female, then use the word “feel.” If your audience is predominately male or motivated more by their thoughts than their gut instincts, then use the word “think.”