A nuclear family is a family group consisting of parents and their children, typically living in one home residence. It is in contrast to a single-parent family, a larger extended family, or a family with more than two parents. Nuclear families typically center on a married couple which may have any number of children. There are differences in definition among observers. Some definitions allow only biological children who are full-blood siblings and consider adopted or half- and step-siblings a part of the immediate family, but others allow for a step-parent and any mix of dependent children, including stepchildren and adopted children. Some sociologists and anthropologists consider the extended family structure to be the most common family structure in most cultures and at most times, rather than the nuclear family.