Frequent hand-washing is the usual cause, but it is also part of normal ageing. Manicures, nail polish and remover, nail biting, and repeated trauma such as typing, can contribute to nail splitting. Dehydration likely plays a role. Nutritional deficiencies that can result in nail splitting include iron, selenium, and zinc. Some skin diseases such as psoriasis and lichen planus may feature such nails.
Onychoschizia
Onychauxis
Koilonychia
Cryptotia
Onychophosis
Auricular hypertrichosis
Platonychia
Hypertrichosis cubiti