[roughly]

The article notes

> The algorithm could generally tell what a person’s political orientation was with a high degree of accuracy, even when that person’s identity was “decorrelated with age, gender, and ethnicity,” researchers write.

But the whole thing is looking at photos - how do you decorrelate a photo from age, gender, ethnicity, or weight?

[shkkmo]

You could read the study and find out.

The simple version was is that there are statistical methods that allow you to control for known factors in wheb doing statistical analysis.