I heard this recently from a friend who claims every hair stylist they've ever spoken to tells them this. After some searching I've found several articles claiming it and several more claiming it's nonsense, but I didn't find any studies. So the question is, does getting regular trimmings (where a "trim" involves cutting off a very small amount of hair, say less than an inch) once every month or so have any observable effect on the growth rate of your hair? If so, how much? And if not, what is the origin of this idea?
Ideally I'm looking for studies that directly compared hair growth between people who got regular controlled trimmings and people who didn't.
A note about the question marked as duplicate: that question is explicitly about hair on the body other than the head, whereas I'm asking specifically about hair only on the head. I don't think any of the answers for that question apply here.
Sourced articles claiming that regular light trimmings to reduce split ends promotes hair growth:
Split ends are one of the major reasons which inhibit hair growth. If your hair is dry and damaged, then you are bound to have split ends. Trim your hair every 3 months to get rid of split ends and encourage hair growth. Remember – never prick your split ends as it would further aggravate your problem.
You will most likely only need to cut 1/2 an inch at each hair appointment, but if this tip seems counter-productive to your goal, then remember this fact from Danielle Bucco-Regazzi, Farouk Systems Creative Artist, “Healthy hair grows faster than dead hair!”
Being in the beauty industry for more than ten years, I frequently speak with women with long hair who never want to get cuts to keep their length in tact. It’s a complete myth. Letting your hair grow out without ever getting it cut will deter your hair from growing at its optimal rate. All women should get cuts every six to eight weeks to promote healthy hair: The two-month trim routine will benefit your hair health and promote growth by keeping the ends nice and fresh. And, if you suffer from breakage, the split will only climb further up the shaft until it's cut.