There has been no study of topical tretinoin showing hair growth alone, but there have been countless studies that demonstrate topical tretinoin in combination with minoxidil can exacerbate the growth progression of minoxidil. This mechanism is well understood: tretinoin promotes and regulates cell proliferation in the epithelium, and great cell turnover allows minoxidil to penetrate the roof covering of the skin as well as promote the enzymatic activity that converts minoxidil to its active metabolite.
From these studies, we can speculate that there needs to be a growth agent in order to see the promotion of hair growth. In addition, millions use topical tretinoin for skincare, and hair growth on the face, forehead, or neck is not reported in the literature or anecdotally. For reference, I have used topical tretinoin for years on my face and forehead, including areas near my hairline, and I've never experienced excessive hair growth at these areas.
That being said, I would suggest beginning the treatment for skincare, whether it be for acne or anti-aging after consulting a dermatologist. Not only is tretinoin a unique compound that outperforms everything available OTC, but does not diminish in effect after years of use and
beginning treatment early (mid-20s to 30s) is one of the best decisions I have made.