Studies based on race serve only to perpetuate racist ideas, while obscuring the true factors behind the observed variations in results. This is why studies which use race as a category produce ambiguous or conflicting results, while studies which use fine divisions of household wealth or income produce clear results.
Race is not a cause of anything; only a creation of racists.
Studying economic disparities and their effects is understandably unpopular in some circles--especially among the elites and upper classes, for some odd reason. Unfortunately these are the same people who have the most influence on what gets studied.
The danger in studies based on fine gradations of income (unlike, for example, the 50-50 division in studies of students on "free or reduced-priced lunch" compared, not with the richer half, but with students "overall") is that results showing a greater negative effect on families making $0-$4,999 per year versus families making $95,000-$99,999 per year, with the degree of negative results tied directly to the degree of poverty--the trouble with these clear results is that, once we learn them, there is something we can clearly do to fix the problem; namely, do whatever necessary to bring an end to poverty.
The "advantage" (for racists and for economic elites) of race-based studies is that once we draw the conclusion that the browner someone is, the worse off they are, there is nothing we can do about it, except congratulate our less brown selves for doing better (through "hard work," presumably), pity the other "races" for being "inferior" (as "proved" by yet another race-based [racist] study), and throw up our hands in futility, because we think that being "black" or "Hispanic" is what causes these problems, and there is nothing anyone can do about someone being a certain race!!
But the honest answer is we can do something about it. We can give up these few-centuries' old falsehoods about "race," we can stop treating other people as fundamentally different from ourselves (they are not), we can reject the continued use of this racist "race" concept in what should be scientific(!) studies, and we can instead scientifically study true causes (injustice, racism, power imbalances, corrupt economic and political systems, greed) of our problems in ways that will lead to real answers that we can take real actions to address and fix.
Originally thumb-typed in response to the Medical News Today article at http://www.medicalnewstoday.com/articles/279260.php (Nutrition and health 'have greater influence on newborn's size than race').