Typically waxing reduces ingrowns, however if done improperly it can actually be the cause of creating ingrowns. It's TRUE!
In general, in order for the wax to grab AND remove the hair it needs to be at least 1/4" long. If it is shorter than that, the wax MAY be able to grab the hair, but just enough to tug on it and cause it to break off deeper into the follicle.
Keep in mind that hair is being pulled and stretched. Think of it as a curling ribbon that you would wrap a gift with. The ribbon is straight to start, but then you stretch and pull it and it curls. It is this breaking, stretching and curling of the shorter hair that can become ingrown as it struggles to break through the surface.
Additionally, those that tend towards ingrowns typically have dry/dehydrated skin, which holds onto the hair. When you try to remove it, it's like playing tug-o-war, and the skin will ALWAYS win.
No matter how much the client begs you to wax their hair, if it's not at least 1/4" long, opt to have them wait another week or two. Let them know the risks of waxing when the hair is too short and the reward if they wait just a little longer. This is also a great time to make sure they have proper home care to get that skin in the best possible condition to get the best possible waxing result.
Next week, the downside to inheriting all that thick, curly hair from your dad (or mom, or great great grandad).