Black people are either born with naturally curly hair or the regular straight, wavy hair.
The amount of curl, wave, or lack thereof, is dependent on the number of disulfide bonds between hair proteins found in the hair shaft; the greater the number of links, the curlier the hair, and the fewer the number of links, the straighter the hair.
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Jheri Curls were intentionally developed by using various hair creams or by extensively wearing them. They were popular in the 80s and 90s.