The “marriage tax”—the extra taxes that a couple pays if they are marry—probably discourages many young Americans from marriage, according to a new study by the National Bureau for Economic Research.

The study finds that the effect of the “marriage tax” falls most heavily on couples who are struggling economically; for them the negative impact is more than twice what it is for the wealthiest couples. Also, women with children are mostly likely to see the extra tax as an impediment to marriage.

Since social-science research confirms that children raised by married parents are likely to fare better economically, the study suggests that the “marriage tax” tends to perpetuate family poverty.