Default boolean for search engine

What do you think is more appropriate (and intuitive) for a search
engine if the user gives no boolean, "and" or "or"?

I guess my question is, assuming it's a keyword search, and the user
types in "institute paper science", would it be more appropriate to
default to "institute AND paper AND science" or "institute OR paper OR
science".

I'm just sort of curious what other people's take on this might be.

Thanks,
-Ross.