Singers have something of a reputation, and not always in a good way. They don't practise as much as instrumentalists - in fact, many of them confuse the words practice and rehearsal - a cardinal sin in my book.
I know that professional singers would argue that they simply can't - the nature of the instrument prevents them from practising six or seven hours a day.
But I am not talking about professionals, although some of this might apply to some of them, too. I am talking about choir singers, especially the ones that think they are doing you a favour, and aren't as good as they think they are. We have all met them.
I am led to think there is a deep problem here. When people compliment a singer they say "You have such a wonderful voice". This implies that the singer has a god-given gift. What does this do? It makes them lazy.
No-one says to a violinist "You have such a wonderful instrument.". They say that a violinist has a great technique, or that they produce a lovely tone, both of which imply that they have actually worked on it.
I am not a psychologist, but I am a teacher, and I know that a child rarely tries harder by being told they are naturally good at something. One must say, "You have worked hard to be good at this".
Stephen Grosz explores this (amongst many others) in The Examined Life: How We Lose and Find Ourselves. Recommended.
If I ever get the time, I shall look up other sources.