I have thought why the theme song of "Kaitai-shin Show" is "Tanin no Kankei", released in 1973 sung by Katsuko Kanai.
From my point of view, Kaitai-shin Show can be for young people because Taichi Kokubun, Gekidan Hitori, and Kirin, etc. who are popular among young people appear. However, NHK may also want people in their 40s or 50s who were in their 10s or 20s when "Tanin no Kankei" was in fashion to watch it.
I know the song the first time on this TV programme. Now, I can sing it.