Most of the human population have straight hair and only about 15% of people have curly hair.
45% of the human population have straight hair while around 40% have wavy hair and then like I said 15% of people have curly hair.
Over the years curly hair and even wavy hair can sometimes change from curly or wavy to straight hair.
My brother had very curly hair as a baby and toddler and kid and then when he got to 20 years old his hair started turning to straight hair.