I don’t think that millions of women around the world are just experiencing some sort of placebo effect from the henna, many swear by it, even to the extent that henna was the only thing that made their hair to be strong enough to grow long.
But, there are many different types and qualities, and it may not work for everyone.

Good properties of henna that people experience include:

-Thicker hair
-Healthy scalp
-Lovely light reflecting red colour
-Lessens shedding
-Stronger hair

While there may not be a technical reference to henna’s conditioning properties, it works for everyone (I mean purely natural, body-art quality henna; not “henna dye kits” sold in stores with metallic salts in them). Well-mixed henna produces a beautiful red tone on hair and covers grays well, and they are less noticeable when they reappear. It leaves the hair highly reflective and unbelievably soft and manageable. In my henna mixes, I use chamomile tea, lemon juice and yogurt and I do not use shampoo or conditioner during the dyeing process.

I dye my hair with henna, and I have dyed seven others’ as well (including my mom, who got a lot of grays from me) – and the results are the same on every woman, although the shade of red is slightly different. There are a million people who swear by it. Try it yourself or see if you can find someone who’s willing…it leaves your hair in amazing shape with a gorgeous color.

I think simply that not enough thorough chemical research has been done. Henna is terrific.