first child versus later children

[Gordon Wells]

In the Bristol Study of Language Development (n=128) we found a
significant correlation of r= 0.29 (p< 0.001) between child's position in
the family and rate of language development.  First bor showed a clear
tendency to develop more rapidly than subsequent siblings; however,
children born after a gap of several years often made rapid progress -
presumably because, unlike their immediately preceding siblings, they had
their parents' more or less undivided attention, as well as the advantage
of attentive older siblings.

Wells, G. (1985) Language Development in the Pre-School Years.  Cambridge
University Press.

Gordon Wells