Golliwog's post on the WeightWeenies forum sparked an interest into the topic of handlebar widths. He asks if others found narrower bars more comfortable, and after doing some reading on this topic it becomes clear that there are different opinions on this topic. Below I've (briefly) summarised some of the main discussion points regarding handlebar width, and included a few of my own.
A general rule of thumb for (road) handlebar width is that they should be approximately 2cm greater than the bony bumps on the front of your shoulders. The logic is that this handlebar width will account for the natural outwards curve of your arms when riding, allowing for a comfortable natural position.
However, little, if any science has been conducted to investigate if this really is an optimal position. What bearing does handlebar width have to do with shoulder width anyway, your arms can bend/adjust to facilitate different widths. Furthermore have you ever noticed your hands rolling inwards? This doesn't seem 'natural' and may suggest that handlebar widths could be narrower.
If we look back through history, between 1930s and 1970s, bikes had comparatively narrow handlebars. It's been noted by Jan Heine that Fausto Copi was riding 40cm handlebars despite being a larger rider. These handlebars are slightly narrower than modern widths, though they're still much wider than the widths I'm about to suggest.
Lately, I've been riding 36cm handlebars, down from my usual 42cm (c-c). My first ride on them was a mixed bag. During straight flat sections the narrower handlebars just felt fast, and allowed me to get (possibly) very aerodynamic (reduced frontal area, at least according to my mirror). However, I found cornering and climbing out of the saddle quite challenging as I wasn't used to the narrower profile. That said, once I had gotten used to the narrower bars my troubles with cornering and climbing quickly disappeared. I soon had a preference for the narrower bars, and on my other bikes which were still fitted with the 42cm bars I felt as though I was acting like a human parachute.
Looking at the numbers, there's a slight aerodynamic advantage to this change. I'm measuring it very crudely to about 30w, I haven't done the error analysis so I can't comment on the significance though. However, provided the errors are within reason, this advantage is nothing to be scoffed at. In addition, the combination of narrower bars and longer stem makes the bike more stable, and actually surprisingly easy to pilot through bunches. With all these benefits, I would have assumed everyone would be onboard. However, most cyclists appear to be skeptical of these narrower bars.
Some may be reluctance to change their handlebars, which is understandable. The saying 'if it ain't broke don't fix it' comes to mind. People have been racing top level on 40-44cm handlebars for years, why change?
Perhaps it's because the advantages aren't fully realised, or cyclists just don't give enough time to adjust for the narrower bars. If I only tried these bars once I probably would have rejected them as a bad idea. But, if we look towards the track, narrow bars are fairly common and a few roadies are taking up narrower bars, most noticeably Adam Hansen and a few of his teammates.
I've also heard a few other arguments against narrow bars, for example it restricts your breathing. I personally didn't experience this, but everyone is different. A more compelling, less personal argument would be; if it was detrimental to speed, why are more extreme positions adopted in time trialling, track racing and by some triathletes?
Another argument is that it isn't as comfortable for endurance cycling and/or climbing. I personally think Jan Heine covers this issue quite well in his blog, but if anything comfort is quite personal, so if you find narrower bars uncomfortable they may not be for you.
If you're curious about trying narrower bars (there's lots of cheaper options around, like the Deda RHM01), I'd recommend trying them for at least 300-400km before making a judgement on the comfort. It could be that they initially feel uncomfortable or weird because you're so used to wider bars. I'd also recommend dropping the bars a few mm, if possible (if you're already super low skip this step), and if you're trying bars which are 4-6mm narrower I'd get a longer stem too. The narrower bars should help you roll your shoulders, so you'll need to increase your drop and reach to compensate (also note that the effective reach to the hoods is decreased by narrower bars, though this effect is only ~3-5 mm).
In summary, I'd recommend (significantly) narrower bars for improved aerodynamics. The benefits of reduced frontal area also translates well into tight racing. Towards the end of my 'experiment' I began to wonder if I really did want to write this post - if everyone started riding narrower bars I would lose this advantage. But, of course not everyone will agree with (or even consider) the above points, and such it seems likely that the vast majority of cyclists will still be on 'regular' handlebar widths.