The standard heat map showing how a page is viewed by website visitors would suggest that placement on the left side would benefit ad earnings by way of being seen more.

The Adsense heat map (Google it) suggests the same.

About 10 years ago I struggled with this, feeling that placing ads on the right was the more ethical thing to do. (Not placing ads where people frequently look for navigation), and went with that.

In terms of Smart Pricing, the right side might have some financial benefit in terms of reducing accidental clicks, but I don't really know. (Actually, I'm not sure anyone could determine this for sure.)

Thank you @Broadway

The best is not to be fixed into a set pattern. I don't use a sidebar at all on desktop, but my ads switch between the left and right hand side of screen. Given that I have both left and right ads I can compare performance. Also I have two right ads "pages", and one left ad "page".

Compare left ad to right ad (page1)
On desktop my lefty ads have double the click through rate and impression RPM as compared to the equivalent right hand ad (page 1). On mobile where there is no left and right click through rate the CTR is statistically the same but the difference in impression RPM. This suggest that RPM is driven by content factors.

Repeated -- Compare left ad to right ad (page2)
The results are similar.

Both right ad units had similar number of impressions, page-2 ads had about 10% fewer impressions. The left ads had about 5% more impressions than page1 right ad and about 15% more page2 impressions. Getting to the pages in all three case is essentially the same. The "pages" are added to the existing page based on a user's click of a button (eg: read-more type thing).

The only caveat is that this is based on 30 days worth of data, and the left, right placement is not randomized, so there could be content biases.

I would generally follow whatever is the norm in the niche. But personally I follow the money.

Mark-up wise, if coding top to bottom, left to right... placing ads on the right gives them more time to load.

Thank you all for your valuable comments.

Also, depending on how you build your site, this will change the order the ad code is loaded. Not sure if this makes a difference in terms of how the ad slots are filled i.e. best first?. But left would load before the main section.

I've always built my sites so the main section appears first in the html, regardless of the actual layout...

I try to avoid strict repetitive layouts to reduce ad blindness. Keeps the site pages visually interesting.