And this is a big problem with reading too much into the original result or the replication:

They're not measuring forcing yourself to smile for yourself, or even forcing yourself to smile for someone else.

They're measuring being tricked into shaping your mouth like a smile without being aware that you're doing it.

That may be interesting and and of itself, but whether or not intentionally forcing a smile has an effect is an entirely separate question.

There is a difference between a forced smile and a genuine smile. There are a different muscles at play.

If you force a laugh, you'll see muscle movements similar to a genuine smile. A forced smile tends to just use the muscles below the nose.

When you test the difference, observe your emotions. Mindfulness is helpful here but I observe a genuine, immediate difference. Perhaps this can be explained by forces other than "fake it til you make it" but it works for me so I can reuse it when I need to. Now I know that a potential placebo effect means it's as good sugar water but sometimes sugar water is all you need to lift your spirits - a little better than rum and cola.

The forced part is. If you actually choose to smile, then it is not forced, right?

I'd like to meet a person who actually can "smile for themselves" most of the time.

As soon as you've decided to do something about being unhappy - even if it's just smiling - it's reasonable to think you'll be at least a slight bit happier.

Yeah - most people probably doesn't just think "I'm not feeling great, so I'll force my facial muscles to create a smile".

Instead they are likely thinking "Hey, what am I bitching about, things aren't so bad. I have lots of things in my life to smile about, so let's do it.".