What determines the rank of a new soldier rewarded from a mission?

[Amritpal]

I used to be getting major for such soldiers.

Now I am getting sergeant. What determines the new soldiers I am getting?

[morde]

When you look at the Reward before accepting the mission, you can see a text that says:

New Recruit: [Class]
      ([Rank])

It is that [Rank] that tells you which rank your new recruit will be. Recruits won as a reward can be of any rank except rookie, while those recruited through the Barracks are without exception rookies^*.

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^*: If you have learned the "New Guy" skill at the Officer Training School, then all rookies will automatically be promoted to squaddies.