643 Address all correspondence to Duane Harland, duane.harland@agresearch.co.nz What Causes Curly Hair? MARINA RICHENA AND DUANE P. HARLAND AgResearch, Lincoln, New Zealand (M.R., D.P.H.) Synopsis We do not know what causes natural-born curliness in human scalp hair or in the hair of mammals in general. This brief review describes why hair curls, and the cause of the phenomenon is more complex than often appreciated. We then briefly review features that are associated, correlated, or purportedly cause hair curl. INTRODUCTION “Eat up your bread crusts children, so that your hair grows curly.” —Traditional This old British saying was more about food wastage than hair. However, crusts aside, the causes of and influences on hair curl are still not completely understood. Here we briefly review current work in this area and explain why answering the deceptively simple question of “what causes curl” remains a challenge. WHY DO WE CARE? We notice hair. Lack of hair, color, texture, and style of hair provide visual information from which our brains subconsciously extract useful social information, such as individuality and conformity to norms. Hair is a tricky signal of anything at a truly biological level (e.g., health status, fitness, or fecundity) because it is easily customized, and, in addition to clothing, hair is the feature that we most readily change temporarily to express ourselves (1). All of this is an important part of our psychology, especially of physical attractiveness (2,3). Curliness is not only an important component of hair style (4), it also has implications for technical performance, especially for people with very curly-textured hair, because of an increased tendency to catch and tangle (5–7). The fur coats of most mammals also rely on some hairs being curly to fill space and trap air for insulation (8,9). In domestic animals, such as sheep and angora goats, hair curliness (related to properties called crimp and bulk) has implications for the uses and value of their wool and the survivability of livestock in different environments (10). As a biomimetic model for protein-based shape memory materials, curly and straight hair could play a key role in informing the J. Cosmet. Sci., 72, 643–654 (November/December 2021)