Finger ratio is an indication of prenatal testosterone in relation to estradiol. In "2nd to 4th digit ratios, fetal testosterone and estradiol" Lutchmayaa et al report:
Results: A significant negative association between right 2D:4D ratio and FT (fetal testosterone) /FE (fetal estradiol) ratio, which was independent of sex. Conclusions: These preliminary findings lend support to an association between low 2D:4D and high levels of FT (fetal testosterone) relative to FE (fetal estradiol), and high 2D:4D with low FT (fetal testosterone) relative to FE (fetal estradiol).
But it's also a predictor of current testosterone as shown in "The ratio of 2nd to 4th digit length: a predictor of sperm numbers and concentrations of testosterone, luteinizing hormone and oestrogen."
The dimorphism is present from at least age 2 years and 2D:4D is probably established in utero; (ii) high 2D:4D ratio in right hands was associated with germ cell failure in men (P = 0.04); (iii) sperm number was negatively related to 2D:4D in the right hand (P = 0.004); (iv) in men testosterone concentrations were negatively related to right hand 2D:4D and in women and men LH (right hand), oestrogen (right and left hands) and prolactin (right hand) concentrations were positively correlated with 2D:4D ratio and (v) 2D:4D ratio in right hands remained positively related to luteinizing hormone and oestrogen after controlling for sex, age, height and weight.
Testorsterone has a lot to do with personality and therefore you can use the finger ratio as an imperfect predictor of personality.