According to the Arthur Brooks study, conservatives give more than liberals. But in another study a significant percent of people who identify themselves as conservatives really hold liberal views on some issues. And those who identify themselves as liberal don't hold conservative views. In other words conservative propaganda over the years has made "liberal" a 4 letter word, ie labeling people as con vs lib isn't so easy. Also, if I remember correctly, church charity played a role in his findings. More conservatives go to church, therefore they give more overall.
Not sure if you are referring to the Brooks study or not, but that's probably the most known one.