I'm not sure "richer than their parents" alone, is a fair metric for social mobility. If you're born in the top 20% and stay in the top 20% (not advancing higher), is that really a social concern? I mean, Bill Gates kids are unlikely to do better than their parents, but that's ok.

From what I've read, most social mobility metrics are from low quintiles to higher quintiles.

So if in 1984, 50% of children do better than their parents, and that 50% is mostly the bottom half (I have no idea if it is), then you're doing pretty good in terms of social mobility, no? People making below the median are doing better.

And I would also expect that social mobility would decrease as an economy "matures". If you're in a developing country seeing 7-10% GDP growth, then you'd hope mobility is higher than an economy growing at 1-2% per year.