Onychoschizia is the medical term for splitting fingernails. The term onychoschizia includes splitting, brittle, soft or thin nails. Onychoschizia is more common in women. Only very rarely are internal disease or vitamin deficiencies the reason (iron deficiency is the most common).
If the fingernails split, but the toenails are strong, then the cause is likely something you are doing. Basically brittle nails can be divided into dry and brittle (too little moisture) and soft and brittle (often too much moisture).
The usual cause is repeated wetting and drying of the fingernails. This makes them dry and brittle. This is often worse in low humidity and in the winter (dry heat).
The best treatment is to apply lotions containing alpha-hydroxy acids or lanolin to the nails after first soaking nails in water for 5 minutes.
Wearing cotton gloves when performing household chores that involve getting the hands wet is very helpful in preventing brittle nails. If your nails are soft, they may be getting too much moisture or being damaged by chemicals such as detergents, cleaning fluids and acetone based nail polish removers .
Be gentle to you nails. Shape and file your nails with a very fine file and round the tips in a gentle curve. Daily filing of snags or irregularities helps to prevent further breakage or splitting. Avoid metal instruments on the nail surface to push back the cuticle. If the nails are "buffed" do this in the same direction as the nail grows and not in a "back and forth" motion because this can cause nail splitting.