Contrary to Sulloway's predictions, they found no significant correlation between birth order and self-reported personality. As more children in a family survive infancy and early childhood, the continued existence of the parents' gene line becomes more assured cf. Harris concludes that birth order effects keep turning up because people keep looking for them, and keep analyzing and reanalyzing their data until they find them. Hence third-born children are not only third in birth order, but they are also more likely to come from larger, poorer families than firstborn children. The authors argued that the effect emerges most clearly from studies within families. They also did their own study on a representative sample of 6, young men from Switzerland. Robert Zajonc argued for a "confluence" model in which the lack of siblings experienced by first borns exposes them to the more intellectual adult family environment. Paulhus and colleagues reported that first borns scored higher on conservatism, conscientiousness and achievement orientation, and later borns higher on rebelliousness, openness, and agreeableness. The theory further predicts that firstborns will be more intelligent than only children, because the latter will not benefit from the "tutor effect" i. As the mother gives birth to more sons, she is thought to develop an immunity to certain male-specific antigens. Furthermore, at least one study, on the familial correlates of joining a same-sex union or marriage in a sample of two million people in Denmark , found that the only sibling correlate of joining a same-sex union among men was having older sisters, not older brothers. When people are with their parents and siblings, firstborns behave differently from laterborns, even during adulthood. Three siblings from the s. Family size, and a number of social and demographic variables are associated with birth order and serve as potential confounds. There was, however, some tendency for people to perceive birth order effects when they were aware of the birth order of an individual. The claim that firstborns have higher IQ scores has been disputed. One modern theory of personality states that the Big Five personality traits of Openness , Conscientiousness , Extraversion , Agreeableness , and Neuroticism represent most of the important elements of personality that can be measured.