I'm keen to know if there is any research/thoughts on the text formatting of the main call to action which affects the conversion rate optimisation!
The text with all capital letters help the button stand out visually. For a call to action button this is important. There is an article by UX Planet about it, that says:
Use capitalization where you need to shout-out an important message, something which you can’t afford your users to miss.
Apparently, it does work out better!
I just finished running a very simple A/B testing on the main CTA on my landing page trying to see if FULLY CAPITALIZED CTA would work better, and over the course of 2 week-long tests, we see a 100% improvement from the FULLY CAPITALIZED CTA over the Standard Capped CTA.
Why not try it out to see for yourself?
I'm not aware of any papers on it but here a few points:
If a page is littered with capitalisation and maybe other attention-grabbing features, most people will probably steer away from it in the future. It's just annoying and sometimes tedious to read through such a mess.
Using attention grabbing techniques is often a sign of a low-quality high-cost offer - worthless subscriptions, shady offers and such. Depending what you sell and to whom, that might not matter. But doing too much of it may be a bad idea for reaching some target audiences.
Capitalisation in a button or two is probably ok. If you capitalise half a sentence in a paragraph as your cta, people may feel yelled at. If you don't do anything, people may be confused about where they need to click.
Drawing the attention to the cta from the beginning can also lead to high conversion rates among people who don't really want the offer, just don't understand what they are just signing up to. Such people will then feel tricked and might not be loyal customers for long. For businesses which rely on such spontaneous decisions, that may be a risk worth taking - money doesn't stink, after all - but for a business trying to build a reputation, that might be disastrous in the long term.
Also, some businesses are more likely to cater to people who like things in a certain way. So the best advice is probably to orient yourself at how things are usually done in your business. Then think of ways to be better than your competitors, for instance by learning from related but faster moving businesses.
On a side note, if you want to do it scientifically, just let a script vary some parts of your page randomly (in future improvements with as much information as possible to get before the click, so you can filter by country, browser and other factors). Try to keep note not only of the resulting conversions, but also of people who cancel later, if reputation is of concern. The more important you find your reputation, the higher you weigh early cancellations.
One oft-overlooked point to take in to consideration is whitespace. Text in all caps is more blocky in nature which meana it's easier to get a nicely balances padding between the text and th e outer border of the button. To a smaller degree this applies to text with a relatively large x-height and small extenders.
Beyond that, it's mostly the same as pros and cons as header/title text. Legibility vs style/attentiongrabbing.
I was only speaking about this a moment ago. They are strange because they are so important but often contain very text. Seldom see a button with more than four words. I think it depends on your design. What does you design do with headings? Are they capitalised, all uppercase or even (yuk) all lowercase? I believe the capitalisation of these CTA’s should link to another element on the screen.