Research has shown that higher birth order (i.e. being the 4th or 5th child in a family versus 1st or 2nd child) is associated with higher risk of Down Syndrome. In this lecture, we showed research results which indicate that:

a) Birth order is a probably "cause" of Down Syndrome
b) Birth order is confounded by age-of-mother, and age-of-mother explains all or most of the risk of Down Syndrome
c) Birth order and age-of-mother work together (additive) so that both factors influence the risk of Down Syndrome
d) Neither birth order nor age-of-mother have a likely causative relationship with Down Syndrome