Americans should contribute more toward retirement savings, Wharton professor Olivia Mitchell says. The U.S. should look to countries such as Australia, which is phasing in a mandatory 12% contribution rate, she says. "This notion that you can get by on 3% or 4% is very misleading. So the lesson from these countries is, yes, we should be contributing more," Mitchell says. She also advocates a lump-sum incentive for Americans who delay Social Security benefits until age 66.