My mother couldn't even match pitch or rhythm but she sang, I sing constantly as much to my children's dismay, My daughter (studying to be a music teacher ) sings when appropriate but my son refuses to sing. He had the same music education as my daughter, grew up in a family where singing is a family activity, and he can carry a tune, he just won't sing. When he was an infant and I would sing while holding him, he'd put his hand over my mouth until I stopped, perhaps I traumatized him!