Fingernails can split for a variety of reasons. Most are easily remedied through change of habits while others may indicate a serious health condition.
Brittle nails that split, medically referred to as onychoschizia, are more common in women and more commonly indicate an external problem rather than an internal. According to the American Osteopathic College for Dermatology, a good way to tell if the nail problem is external or indicates a health problem is to look at the toenails. If the toenails are healthy and strong then the problem is commonly an external factor.
Nails can become brittle and split when constantly exposed to water and dried. You can remedy this by applying a nail moisturizer and by keeping your nails dry by wearing protective rubber gloves when immersing hands in water. Nails can also be too soft and split from too much moisturizer or exposure to harsh chemicals. Again, the solution is to wear gloves when working with chemicals and to limit your exposure to products such as nail polish remover.
Common health conditions can also cause fingernails to split such as psoriasis, which starts in the nail bed for 10% of sufferers, and thyroid disease. A vitamin deficiency, most commonly iron, can sometimes cause nails to become brittle and split.