Slavery is any system in which principles of property law are applied to people, allowing individuals to own, buy and sell other individuals, as a de jure form of property. An enslaved person is unable to withdraw unilaterally from such an arrangement and works without remuneration. Many scholars now use the term chattel slavery to refer to this specific sense of legalized, de jure slavery. In a broader sense, however, the word slavery may also refer to any situation in which an individual is de facto forced to work against their own will. Scholars also use the more generic terms such as unfree labour or forced labour to refer to such situations. However, and especially under slavery in broader senses of the word, slaves may have some rights and protections according to laws or customs. Slavery has existed in many cultures, dating back to early human civilizations. It was legal in most societies at some time in the past but is now outlawed in all recognized countries. Nevertheless, as of 2019 there were an estimated 40 million people worldwide subject to some form of slavery, 25% of them children. More than half of modern slaves are used for forced labor, mostly in the private sector. The most common form of modern slave trade is commonly referred to as human trafficking. In some areas of the world, slavery continues through practices such as debt bondage, the most common source of modern slaves; serfdom; domestic servants kept in captivity; certain adoptions in which children are forced to work as slaves; child soldiers; and forced marriage.