What a great question.
There can be all sorts – you probably have a time limit as you might only be funded to do the research for a certain amount of time.
Then you’re also constrained by the ethics committee of your University, and of your funder. You have to get ethical approval for every experiment you run. This is obviously really important, and definitely a good thing, but it can constrain what you can find out.
You can also be constrained by the type of people (in my case) that can do your experiment. Apart from in very rare circumstances people have to volunteer to do your research, so this can constrain your experiments.
I’m sure there are lots of other things that can constrain your experiments too.
Thanks for the question, it’s a really interesting one!
I think Suzi has mentioned the main ones. Most of the contraints just require you to think very carefully about how you design your experiments. For me time is the hardest contraint, not the time the project takes, but the time an individual experiment takes. When you measure the brain activity of young children you can’t expect them to sit still for very long, you have to design the experiment to be fun and as short as you possibly can.