I'd be curious to see if conversions went down, as the average price went up.

People who only wanted to donate $12 may have seen the frown, and decided "This amount isn't wanted", and left, rather than increasing their donation?

If we assume some people did each (some leave, and some increase), the adjusted total donation numbers with and without the smiley, would be interesting to compare, not just the average donation amount.

Did the smile change in appearance (e.g. look happier) as the slider went up the scale?

It seems you have another sweet spot at the $36 mark, despite the smile looking the same as $24.

Thanks for the insightful post.