There are many mechanisms for how castor oil products can make facial hair thicker.
Castor oil contains 85-95% of a special type of monounsaturated fatty-acids called Ricinoleic acids, and this type of fat has been scientifically proven to increase hair density in laboratory animals by stimulating the production of prostaglandin E21,2, which is known to promote the thickness of the hair spewing out of follicles3.
Other similar monounsaturated fatty acids (found in olive oil and argan oil) have been found to increase testosterone levels in men who consume them orally.
This isn’t proof of castor oil actually increasing testosterone when applied to the skin, but at least it shows that it should theoretically have a positive hormonal effect in humans (instead of negatively affecting hormones like many polyunsaturated fats found in poorly designed beard oils).
The third mechanism of action is not one of those natural methods that stimulate new growth of facial hair, but instead, a way to make your beard appear fuller visually.
Castor oil is denser than most oils and it creates the visual impact of a thicker and denser beard, that in combination with its slight circulation boosting effect makes castor oil the perfect daily growth stimulating beard oil.