I can't speak to the actual research but after reading various declassified documents from the Soviet Union and Maoist China, I can say truthfully that they did not believe the world would end with a nuclear exchange. Both countries had 1st strike scenarios - both believed some aspect of their government and country would survive.

Mao was particularly disconcerting, to paraphrase, "Nuclear war doesn't scare me, we've got more than enough people and cities, we can rebuild"

Upon further looking into it, I fear MAD may have been at the time an overexaggeration to prevent what would be the most devastating war. Could be true now tho - things have leveled up