The Sternocostal Head
"One of the most common assumptions in the world of iron is that the decline bench is the best for developing the lower pecs. However, this familiar premise may be nothing more than another unfounded gym myth. According to the Barnett EMG study, the flat bench produced much more electrical energy in the lower pecs than did either the decline or incline positions. "I agree with this research" says NPC National Champion and pro bodybuilder Jay Cutler, "The flat bench is much better for lower pec development than the decline."
Barnett, C., Kippers, V., and Turner, P. (1995). Effects of variations of the bench press exercise on the EMG activity of five shoulder muscles. Journal of Strength and Conditioning Research. 9(4): 222-227.
Decline presses do not stimulate the sternocostal head of the pec better that flat benches. The myth was that there is an upper chest, lower chest, and middle chest. The pectoralis major is one muscle with two heads: clavicular and sternocostal. Incline press hits the clavicular head better than flat, but flat hits the sternocostal head better than both decline and incline. The necessity of incline presses is debatable amongst lifters and their overall response to stimulus.
^ This is correct and the cause of this is the emphasis placed on pressing motions like flat bench, incline bench, weighted dips, overhead press and even pullups. Pullups focus on development of the lats and the lats are part of the internal rotators of the humerus, which only contributes to the muscular imbalance. Emphasis on rowing, depression of the scapula, face pulls and external rotation motions will provide beneficial to the condition.