Birth order

This article describes a measure that can be evaluated for a particular individual person, household, or family. It is not a statistical aggregate, though it may be statistically aggregated to provide average measures and distributions.
The measure is a lifetime constant, i.e., it does not vary with time.
The measure is not subjective, as the measurement of its value is not based on people's beliefs, opinions, and/or values. Note that the actual value itself is influenced by people's decisions, and hence by their subjective preferences, but the measurement of these values is not.

Definition

The birth order of an individual is defined as the rank of that individual among his or her siblings, where older (earlier-born) siblings are ranked higher.

Note that since the rank is defined as oldest first, the birth of future siblings does not affect a given individual's birth order. Thus, an individual's birth order is determined at birth.

Possible complications in the definition

Complications arise from:

• Half-siblings
• Birth parents versus adoptive parents
• Twins or other forms of multiple births
• Some siblings dying before or after the birth of others

Related notions

• Birth spacing

Particular cases

Birth order	Common language jargon
1	first child, oldest child
2	second child, second oldest child
3	third child, third oldest child