Does training legs make your arms grow more too?

Short answer

No. When the two arms of the same people were compared for 15 weeks, the arm trained with added leg work grew 10% and the arm-only side grew 12%: no difference. People whose hormones rose more after training did not grow more muscle either.

Does training legs make your arms grow more too?
'Leg training → hormones up → arms grow': is this formula right?
A common claim is that big leg lifts like squats raise testosterone and growth hormone and help the arms grow too. That leads to advice to add legs on arm day. We checked whether hormones rise, and whether those hormones make arms grow.
Hormones did rise after leg training
But the rate of building arm muscle did not change
8 people trained in two ways. With arm training alone, hormones stayed flat. With heavy leg work after the arm training, testosterone and growth hormone rose sharply. But muscle samples from the arm showed no difference in how fast muscle protein was built.
Two arms of the same people, 15 weeks
10% and 12%: no difference
12 people trained one arm alone and the other arm followed right away by a large amount of leg work, for 15 weeks. On leg days, hormones rose sharply for all 15 weeks. Still, arm muscle size grew 10% on the leg-added arm and 12% on the arm-only side, with no difference. Strength gains did not differ either.
Whole-body training vs arms only
No difference in arm size measured by MRI
17 people trained for 10 weeks, either all the big muscles of the body or the arms only. Across both groups, arm size measured by MRI grew 12% and arm strength rose 30%, with no difference between groups. Whole-body training added nothing extra to the arms.
What about results where legs helped arms grow?
They came from only two studies, of 9 and 20 people
9 people trained one arm right after leg exercise for 11 weeks, and only that arm grew at its thickest part and gained more strength. Muscle volume grew in both arms. In a 6-week study of 20 people, the group doing high-rep leg training gained more bench press strength and arm muscle area, but the two groups also trained their legs in different ways.
Did people with bigger hormone rises grow more?
49 and 56 people: no link
In a 12-week study of 49 people, how much hormones rose right after training was not linked to muscle or strength gains. In a study of 56 people, those whose testosterone rose more did not gain more lean mass (muscle and everything except fat). Instead, the stress hormone cortisol showed a weak link.
Summary: arm training builds arms
The hormone rise after training did not change arm growth
Leg training raises hormones, but those hormones changed neither the rate of building arm muscle nor arm size after 15 weeks. Bigger arms showed up only in two studies, of 9 and 20 people. Arms grow from arm training, and leg training is for the legs.