Source?
Based on quick analysis of my SP500 data, since 1993 the open is only 54.8% higher than previous close for SP500 with average gain of 0.0363% gain.
For the last few years:
2011-to-date: 52.34% higher opens, avg gain = 0.0215%
2010: 52.38% higher opens, avg gain = 0.0271%...