"This kind" being the research we've been discussing, which is about socialisation and toy preference. Finger length and other easily-measured physical differences haven't been part of it.
It's briefly mentioned in the article as a possible indicator of toy preference because it correlates well with hormone levels. It clearly wasn't what the research, the discussion, or any of my comments were focusing on.
It's reported in all of the child studies discussed in TFA. Fault the article if you want but "hormones" is in the title, and the whole point of the entire line of research is that it's implausible that prenatal hormone levels could be affected by cultural biases.
> Assuming a normal distribution, the 95% confidence interval for average length is 0.889-1.005 for males and 0.913-1.017 for females.
> In Manning's words, "There's more difference between a Pole and a Finn, than a man and a woman."
> 2D:4D is being used alongside other methods to help sex Palaeolithic hand stencils found in European and Indonesian caves.