Does focusing on the muscle while lifting make it grow more?

Short answer

For the arm, yes. Over 8 weeks, the arm muscle thickened by 12.4% when people focused on the muscle and by 6.9% when they focused on lifting the weight. The thigh showed no difference, and the focus effect vanished for the chest at 80% in the bench press and for the back in the last reps of a set.

Does focusing on the muscle while lifting make it grow more?
Focus on the muscle → it works harder → it grows more. True?
A common tip is to focus on the working muscle rather than the weight. We checked step by step whether focusing really makes that muscle work harder, and whether it then grows more.
Focusing made that muscle work harder
At light to moderate weights, it did
When 18 people focused on the chest or the back of the arm during the bench press, that muscle's activity rose at 20~60% of the heaviest weight they could lift, but not at 80%. When 31 people did a machine shoulder press at 40% of their heaviest weight, focusing on the shoulder raised shoulder activity, and focusing on the back of the arm raised that muscle's activity.
No effect for the chest or at the end of a set
The chest did not rise even at 50% in two 13-person studies, and the back effect vanished with fatigue
In a bench press study of 13 people, focusing raised activity in the back of the arm at both 50% and 80% of the heaviest weight, but chest activity did not rise at either weight. When 13 people focused on the chest over 4 sessions at 50%, chest activity did not change either. When 20 people rowed to failure, back muscle activity rose by 15.21% in the first reps and showed no difference in the last reps.
So did the muscle grow more?
The arm grew more; the thigh showed no difference
30 people trained 3 times a week for 8 weeks, doing 4 sets of 8~12 reps per exercise. Those focusing on "squeeze the muscle" gained 12.4% in front upper-arm muscle thickness, versus 6.9% for those focusing on "get the weight up". Front thigh muscle thickness grew similarly in both groups.
In reverse, strength was higher with an outside focus
Focusing on pushing the bar made people stronger
In an analysis that pooled 10 studies, people were stronger in a single maximal effort when focusing on pushing the bar or the floor rather than on the muscle. Over 6~12 weeks of training, strength gains showed no difference, and in lower-body exercises the outside-focus group gained more strength.
Practice made people better at targeting
Only bodybuilders used the chest more on cue
When 37 people bench pressed at 60% of their heaviest weight, only the 10 bodybuilders used the chest more when focusing on it; powerlifters and people who did not train did not. In a push-up study of 18 people, chest focus raised chest activity by 9%, and the ability to target the back of the arm grew with years of training.
Summary: the arm muscle grew more
Muscle focus for light isolation work, bar focus for heavy lower-body lifts
Focusing on the muscle raised its activity, and in an 8-week study the arm grew more, 12.4% versus 6.9%. The thigh showed no difference, and the effect vanished for the chest at 80% in the bench press and for the back at the end of a set. Focusing on the muscle in arm isolation work, and on pushing the bar when going for strength in heavy lower-body lifts, is what works.