I suggest some commas (to avoid tendency to read "average developing countries" as a phrase) and moving the word "yet": This suggests that , on average , developing countries have not yet reached their steady-state growth path. I question whether "on average ... have not reached" is quite logical.
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PvunderinkIs "This suggests that most developing countries have not yet reached their steady-state growth path." better?Do I need a comma?If that's the case then yes, that seems better. No comma needed.