Ned Davis Research indicates that rate of change in gas prices is a critical factor stock investors's outlook: when prices at the pump rise more than 30% within a year stocks often slide. Conversely, when gas prices have decreased over a 12 month period, the S.& P. 500 has averaged an increase of about 13%.
Interestingly, in early May 2011 a gas averaged $3.96, which was about 35% percent higher than May 2010. In 2012, gas has averaged $3.83 per gallon which is less expensive than a year ago. Hence, one might conclude that stocks might avoid the "sell in May" condition.
Source(s): Ned Davis Research, NYT