Birth order refers to the place of a child's birth in the family. Children are either "first born " or "later borns".
The influence of birth order is still an open issue, but some clear patterns have been established.
For males the number of older brothers (born to the same mother) increases the chances of homosexuality. With each older brother a male's chances of being a homosexual increases by about 30% relative to a first born male. The developmental mechanisms of this phenomenon are unknown, but it is theorized that immunological responses to subsequent male fetuses by the mother's body may affect certain neurological masculinizing effects of testosterone. No such correlation between birth order and female homosexuality exists.
Personality too is clearly and strongly influenced by birth order. Personality psychologists largely (though by no means without debate) agree that the Big Five (aka Five Factor) personality traits represent something like a natural taxonomy of human personality variables. Cross-linguistically the vast majority of adjectives used to describe human personality fit into one of the following five areas, easily remembered by the acronym OCEAN: Openness, Conscientiousness, Extraversion, Agreeableness, and Neuroticism.
In his book "Born to rebel", and in ongoing research papers, Frank J. Sulloway has mounted conclusive evidence that birth order effects on the Big Five are strong and very consistent. Using a scale between bipolar adjective pairs (ex.: hard-working . . . . . . . lazy) and intrafamily ratings with tens of thousands of respondents Sulloway has shown firstborns to be more conscientious, more socially dominant, less agreeable, and less open to new ideas compared to laterborns. In a cross-cultural replication of Sulloway's work anthropologist Paul Roach collected several hundred within-family ratings among horticulturalist Shuar Indians in Ecuadorian Amazonia and found nearly identical correlations between the Big Five and birth order among these nonwesternized people (conference presentation: HBES, 2002, Rutgers University).