Just something to add to this interesting topic, it may be irrelevant but the standard key for a harmonica tends to be C. As with Cajun accordion any instructional CDs feature a C harmonica and those beginner sets you can buy come with an instrument in C.
A is a popular blues harmonica key but C is considered the all round starter key.
Apparently if you go to a Cajun style jam in France the preferred key of accodion is D.
Consider the choices in musical keys A,B,C,D,E,F,G. Which key is exactly mid range? "D"...with 3 keys before it and after it. So why not choose to manufacture midrange "D" accordions? Makes sense to me. Not to mention how they would fit like a glove together with standard tuned (G,D,A,E) fiddles.
Nedro and others, let me point out another reason that the D accordion
might have been right for the early cajun singers.
Cajun men were, in general, smaller
than their grandchildren and great grand children are now at a comparable age.
A cursory study of the anatomy and physiology of the larynx, vocal cords,
and would suggest that the resultant smaller structures in smaller men would
change the acoustic properties of the resonating chamber. A smaller
larynx would naturally create a higher pitch.
John, your scientific analysis holds water. Small fellows with high voices. Tours of the River Road plantations near New Orleans point out that the French Caucasians were quite small, possibly 6 or 7 inches shorter than the present norm.
250 years of gumbo, boudin, and jambalaya have doubtless contributed to the dimensional assimilation of the Cajuns.