The results of the poll so far:
Oldest 40% [ 34 ] x
Middle (second, third, etc..) 13% [ 11 ]
Youngest 19% [ 16 ]
Twin, triplet... 5% [ 4 ]
Only child 21% [ 18 ]
Other 1% [ 1 ]
Total votes : 84
Then so far 54 of the respondents out of the 84 were actually "first born" children:
(Oldest as firstborns = 34, half of the multiple births were first born = 2, only children are also first born children = 18 = a total of 54, a surprisingly large number on this informal WP survey).
I remember reading somewhere that a study found a preponderance of firstborns with AS that was statistically significant, when the firstborns were compared with their own younger siblings. There may be something in the theory of firstborns being more likely to be on the spectrum, though as children of all birth orders appear on it too, being first born cannot be causal.