single click opens mail?

When I am using Mail and I click on a new mail item one time (intending to view it in the lower half of the panel) it opens in a new window? I do not want it to open the message in a new window unless I double click it. Which is what I would ordinarily expect and is how it works on my other mac book pro. I have not been able to figure out how to correct this. I have looked at the mouse settings (I am not using a mouse so none available) and trackpad settings (none seem to apply) as well as keyboard settings (again, none seem to apply) to no avail. Perhaps the clicker (selection bar? I am not sure what it is called) is broken? Thank you for any assistance you can provide. MACarmel!