People give when they're asked.
Jim Andreoni and Justin Rao have proved it. They ran the following experiment: one person, the allocator, was given 100 'money units', worth $10 in real money. She was free to choose how much to keep for herself and how much to give to another person, the recipient. The recipient, however, had an opportunity to ask for a particular division of money - 50/50, say, or 30/70 or 60/40.
It turns out that people who ask for more get more - up to a point. When the recipient asks for, say, 70 percent of the money in the envelope, the allocator is quite likely to say "sorry" and give nothing. But a recipient who asks for a 50/50 split on average receives more than the recipient who asks for nothing.