Why do beards appear to grow back thicker after shaving?
This is a die-hard myth. But think of the hair as a rope.
A rope with a diameter of 1 centimeter and a length of 3 centimeters has a thickness to height ratio greater than one with the same diameter, but 6 centimeters long.
This makes the higher string appear less rigid and more flexible, since the same force over a longer length is coupled with a greater mechanical moment, and there are more portions of the string that can be moved.
This is why cutting hair short makes it look thicker, and those with thin hair have less volume (as the thickness to length ratio decreases, they tend to sag under their weight).
The hair is also thicker at the base and thins out as it grows.
And as if that weren’t enough, beard hair, being androgenic hairs (the thick hair that grows on some parts of the body during puberty) is already very thick.
People who trim their beard frequently see only the thickest part of a shorter hair. And those kinds of hairs are already pretty thick. The beard therefore appears smoother and harder, but just wait to see that it hasn’t really become so.