Afghans, particularly Pashtuns, are noted for their tribal solidarity and high regard for personal honor. One writer considers the tribal system to be the best way of organizing large groups of people in a country that is geographically difficult, and in a society that, from a materialistic point of view, has an uncomplicated lifestyle. There are various Afghan tribes, and an estimated 2–3 million nomads. Afghan culture is deeply Islamic, but pre-Islamic practices persist. One example is ''bacha bazi'', a term for activities involving sexual relations between older men and younger adolescent men, or boys. Child marriage is prevalent in Afghanistan; the legal age for marriage is 16. The most preferred marriage in Afghan society is to one's parallel cousin, and the groom is often expected to pay a bride price.