What causes fingernails to curl down?

Fingernails can curl down due to several factors, including genetics, aging, or underlying health conditions such as psoriasis or thyroid disorders. Nutritional deficiencies, particularly in vitamins and minerals like biotin or iron, may also contribute to changes in nail shape. Additionally, frequent exposure to moisture or chemicals can weaken nails, leading to deformation over time. If curling is sudden or severe, it's advisable to consult a healthcare professional.