That depends on the program. Here's my situation. I'm doing my residency at Ohio State in Columbus, OH. They are implementing (starting with my class) a required 1 year research stint for 1 out of the 4 residents in each class. This is to be done after the 3rd year of training. The hope is that one of us will volunteer for the year, if none of us do then they'll choose one of us to do it. Hope it helps.

Varies from program to program. Several (Penn, Pitt, UNC, etc.) have research positions designated with each resident group (2/8 for Penn, 4/8 for Pitt, etc). You have to rank the postions with/without a research year separately. Some programs select the research residents from all those accepted. Pitt formerly did this, but they don't any longer, and I'm not sure who still does this. In Pitt's case you would show up for your first day, and they would tell you then if you were there for 5 or 6 years. To my understanding, at least with the Eastern US programs, if you are asked to do research you have no option to decline.

Louisville selects one research resident per year outside of the match. The year is done immediately following the intern year.

At CWRU, 2 of the 6 residents go into the lab after intern year. They ask you about your research interest during the interview, but I don't think your answer has any bearing on whether you are accepted here for residency or not.

As to who goes into the lab: They leave it up to the interns to decide amongst themselves (before they start intern year) as to which two will be going into the lab. Most years there's usually 2 out of the 6 that want to go into the lab. I'm not quite sure what happens when no one does. Maybe they do rock-paper-scissors or something =).