Many people spontaneously start to sing in the shower. This
predisposition to song is a happy mental state which many people are
familiar with. Singing or humming is entered into almost unconsciously
(rather than from deliberate volition), reflecting the contented
neurochemistry the shower is somehow able to create.

Clearly, there must a factor in the shower that perhaps alters
physiology or brain state, subtly predisposing an individual towards
entering into song. (For me, taking a bath does not evoke the
spontaneous song effect - unless I use the bath's shower head - so I
believe the factor or factors relate specifically to the flowing
shower water.)

I would like to know what evokes the spontaneous singing effect: has
any research been conducted on this phenomenon?

Something quite profound may be happening here. This mental
disposition to song may indicate a switch of hemispheric dominance
from the left brain (language and logic) to the right brain (music and
verse). One imagines that there might even be a measurable change in
brain EEG rhythms whilst in the shower.

Here are my speculations. These speculations may or may not be correct
(and I provide reasoning why these speculations probably fall short of
the truth), but they are a starting point, and indicate in general
terms the sort of answer I am seeking.

Several possible influencing factors spring to mind: (1) negative air
ions are created by cascading shower water, and these ions are know to
effect mood (though I note that I never burst into song when I switch
on my negative air ion ionizer); (2) The air humidity rises with the
steam from the shower, and this may have some physiological action on
the lungs, or this humid heat may increase peripheral blood flow
(though I never find myself singing just when the weather is hot and
humid); (3) The dense steam may help clear the nasal passages, which
in turn may stimulate the brain (the nasal sense feeds into the
olfactory bulb in the brain, and anatomically, the latter is closely
linked to the brain's limbic system (the emotional centre), so
something like this is a possibility; (4) The pleasant tactile
sensation of hot water running down the body may alter mood (we
usually become relaxed with massage and similar skin stimulation - but
rarely burst into song during a massage); (5) In addition to steam,
chlorine gas from chlorinated water also enters the air (though I
doubt that this is a contributory factor); (6) Then there are some
intangible psychological factors, such as the fact that one is in a
private environment (but in my case, I never catch myself
spontaneously singing just because I am alone in private).

One could conceive of many other possible factors, with a bit of
thought and imagination. The spontaneous song effect may also be the
synergistic result of several factors operating in tandem.

The correct factors, whatever they are, however, are the ones that
will create this predisposition to spontaneous song even when
artificially applied to a subject, outside the shower, in any
environment. The song effect must be reliably created whenever these
factors are administered.

My question is: what exactly are the causal factors behind the
"spontaneous song effect" experienced in a shower, and if this
question cannot be answered, what research has been done towards
discovering their nature? Any anecdotal speculation will also be of
interest to me.

I think the shower-singing phenomenon occurs largely because of the
acoustics. The reverb effect of being in a small enclosed area with
hard, sound-reflective wall surfaces makes most people's singing
voices sound better.

I used to have a cat named Elvis Pussley who sought parts of the house
where his voice reverberated (usually a narrow hallway), and there he
would sit and caterwaul at length, in the pretense that he was a wild
old tomcat having a howl.

This might interest you:
http://www.exploratorium.edu/music/questions/shower.html

Who is going to argue with Pinkfreud and Bobbie7?
Empirical and web research in perfect harmony!

I once tried to figure out if my glass and ceramic showerstall or
maybe the whole small bathroom had better harmonics in one or the
other key.  Since I am not much of a musician, the conclusion was
indefinite:  I think so, but I don't know the key.

Pink, if someone named me that, I would certainly try to find the
place where I could make the most ruckus.  Have you read T.S.Eliot on
naming cats? (of course)  see the question on the subject.  ;-) Myo

I will attempt to explain on the basis of our evolutionary history
(Darwinism, not intelligent design).
Humans like to sing / play music when they are happy or relaxed. Lot
of research has been done on that. If you google terms like
"evolution" + "singing" + "reasons" etc., you will get plenty of hits.
Now for the part why we tend to sing while taking a shower, the
explaination is just my hypothesis, I found no related literature
online.
Probably shower mimics the effect of rain. In pre-historic times rain
should be a welcome event. Ending drought, ensuring there will be more
fruits to gather, more grass and hence more animals to hunt etc. And a
free shower of course.
In countries where rains are a seasonal phenomenon, like in India,
there are still many festivals associated with onset of rainy season.
So I think, shower mimics rain, makes us feel happy and relaxed, and
we break into singing.