In academia, is there evidence that including phrases such as "refreshments provided" in a seminar announcement increases the average attendance (even slightly)?
I would love to be able to say how content matters the most (I still believe it does), but in my experience in several academic institutions, providing refreshments is welcomed by grad students and faculty alike. If there are two events with equal content quality, one of which offers food, it will most likely get increased attendance.
I think you should approach the question slightly differently - don't ask whether more people will come to your presentation because there are refreshments. Instead, ask whether refreshments will enhance the overall experience. In my experience, refreshments help to keep the group together after the event is over, which in turn allows for people to mingle, share ideas, and get to know one another better. When refreshments aren't provided, people tend to bolt out the door immediately following the end of the lecture. In this sense, refreshments give you the chance to build community, and give undergrads, graduate students, and junior scholars the chance to bend the ear of a senior scholar.
My own experience is that it depends mainly on the level you are dealing with.
- With graduate students, postdocs, and faculty/staff, it is not so much that refreshments attract people, but that when refreshments are customary in a community that a lack of refreshments will disappoint attendees. I have known highly attended seminar series that never have refreshments.
- With undergraduates, refreshments make a big difference, especially cookies or pizza.
It works for best 'newcomer' group among researchers.
When I was a new grad student, I was enthusiastic about the 'refreshment' part. Often attended programs for it.
Gradually, we grow out of it. Now, free pizza does not appeal as much as it used to do 3 years ago. Drinking sugary drink was much more fun without guilty feelings (I was slightly younger, and cared less about my body that carries the head that contains the grey matters)
I don't see any reason that offering food could repel people.
But, if the seminar is not attractive by himself, offering food may be of no help. I remember that, few years ago, my university decided to have a (very unformal, simple) cocktail after our general colloquium. It had almost no effect on the attendance to the seminar: most absentees didn't come to the seminar because it was at 5PM and they wanted to be home early. Offering crackers, cheese and good wine (that was in France) and orange juice didn't change anything for them.
If you are doing a seminar where you expect folk to travel to hear it, then I have found (for professional engineering (UK) seminars) that a nominal charge will help folk actually turn up (things for free aren't worth anything are they; Are they?).
A small fee also allows folks to 'justify' their attendance and ancillary expenses that would not be allowed for a free seminar (a flight, a hotel, etc). It's a case of knowing your target audience and their issues and concerns, even contradictory ones!