I pulled up at a stop light even with a young woman in the next lane. Though the window was closed, I could see that she was singing moderately loudly. Is there something inherently social about music that bids us to sing along? It can be suppressed, certainly, but so many of us do it even when alone that I conclude there must be something natural about it, something evolutionary. We can all watch a performance, but when those around us sing there is some movement toward joining them, at least at some level of a hum. It could be learned, as we do teach children to sing a long with us for both instructional and social reasons. Yet I think not. Not all families teach their children to sing.
It is not universal. There is a continuum, and I am at the far end of it, rumble-humbling a bass line to everything I hear, though often audible only to myself. I hope to come back as a cello in my next life. Or perhaps I was one in a previous life. It is one more place where I have few filters, happily or sadly.
There are those who have so little interest in music that the fit does not take them and they ignore what is going on about them with impassive faces. Though not always. I have caught a few of those singing in the most surprising places.