Why do conservatives donate more money to charity than liberals?

A book published in 2006 by economist Arthur Brooks, a professor at Syracuse University, detailed the evidence that conservatives are more generous than liberals. They are more likely to donate money, time, and even blood.

While most of the sources I can find discussing this have a decidedly conservative slant (George Will, John Stossel, Human Events), the data are the data, and it does not surprise me that liberals are not jumping at the chance to discuss this. In fact, I was surprised not to find a thorough liberal debunking, which gives this even more traction in my mind.

So unless you can provide a source showing that this is not factually accurate, let’s proceed on the assumption that it is. What I’d like to know is: why is this the case? I was under the impression that conservatives generally thought the free market will take care of everything and that if people are poor it’s because they’re lazy and they deserve it. So why are they so eager to help out the downtrodden?

Is this a wake-up call to liberals? Should we be more conscientious in our individual efforts to help the less fortunate?