Suppose you learn that rich and poor people get drunk equally often.  Should you conclude that the poor are no more prone to irresponsible drinking than the rich?

No.  You can’t sensibly categorize behavior as “responsible” or “irresponsible” until you know the actors’ circumstances.  The greater the risk your behavior will lead to dire consequences for yourself, your dependents, or bystanders, the more irresponsible your behavior.  The richer you are, the easier it is to avoid or remedy such consequences – and the less likely a given action qualifies as irresponsible. 

Consider these obvious cases:

1. Spending $100 on dinner.  This is extremely irresponsible if you only have $105 to your name, but fine when you’re a millionaire. 

2. Having unprotected sex.   This is irresponsible when you’re unable to support a child, but fine if you’re prepared for parenthood. 

The same logic holds for drunkenness.  Heavy drinking has well-known health and employment dangers.  The poorer
you are, the less able you are to cope with these dangers if they materialize – and the greater your obligation to avoid taking chances in the first place.

The upshot is that if behavior does not vary by income, we should conclude that the poor are more irresponsible than the rich.  If the rich actually engage in less risky behavior than the poor, the true gap is bigger than it looks.

If you’re outraged by this implication, note that family status works the same way.  When a childless single courts danger, he risks his future.  When a married parent courts danger, he risks not only his own future, but the
future of his spouse and his kids.  Think about riding a motorcycle.  This could simultaneously be a reasonable trade-off for a childless single and a reckless gamble for a married parent.  Why?  Because when you’re a married parent, the total downside is much more serious.

Aren’t family status and income fundamentally different?  Not really.  Neither depends on choices alone.  Opportunities and luck both play their role.  The virtuous path is not to bemoan our situation, but to act responsibly in whatever situation we find ourselves.