>[[User 'Colin Fine' 's answer:]](https://english.stackexchange.com/a/1063/50720) Note that **so** here [in **so do I**] has a different meaning from that it would have in the normal order ("I do so") - [the inverted **so**] means 'also', or 'as well' and it seems to me that it can have that meaning only when fronted. The fronting is clearly for emphasis, but that in itself is not enough to explain why the fronting is obligatory for that meaning.

So why's subject-auxiliary inversion necessary in such elliptical sentences? I doubt or don't understand the other answers in that question, such as [this](https://english.stackexchange.com/questions/1049/why-do-you-say-so-do-i#comment125306_65195). 

Footnote: [Wikipedia](http://en.wikipedia.org/wiki/Subject%E2%80%93auxiliary_inversion#Inversion_in_condition_clauses) doesn't appear to explain this necesary inversion.