The decline bench press is the best way to focus on the lower pecs.
The bench press adequately hits the lower pecs without necessitating the addition of decline bench presses.
The two main heads of the pecs start on the rib cage and collarbones, and converge onto one common tendon that attaches to the humerus (upper arm) bone. The clavicular head mainly starts on the collarbones and is known as the upper pecs. The sternocostal head mainly starts from the sternum and makes up the middle and lower pecs.
Research from Australia comparing bench presses to decline bench presses found that trained subjects were more than 15% stronger on decline bench presses than bench presses. This allows you to place more overload on the pecs.
The Australian researchers also found that the muscle activity of the sternocostal pecs was actually 25% less on decline bench presses than on bench presses. The muscle activity of the lats was almost 100% greater on decline bench presses than on bench presses. This means that decline bench presses may place a little less focus on the lower pecs and more focus on the lats than bench presses.
VERDICT: BENCH PRESS
The bench press places more focus on the sternocostal head of the pecs (the middle and lower pecs) than decline bench presses. However, decline bench presses allow you to use slightly more weight than bench presses, which can help to place more overload on the pecs.
The bench press should be the major exercise you use to build up the lower pecs. However, occasionally doing decline bench presses will allow you to go heavier, so you can place more overload on the lower pecs, which can also lead to greater muscle growth.