"I agree with you that the OP is making a wrongful assumption that intelligent mothers will breastfeed against the odds"
But length of breastfeeding does correlate strongly with social class and education.
The women who breastfeed longest tend to be the best educated.
Assuming that breastfeeding problems such as mastitis, issues surrounding supply, nipple trauma etc affect all women equally across social classes, it does seem that better educated women are less likely to stop breastfeeding because of any difficulties they might face.
Re: work issues, professional women are both the most likely to return to work, and also the most likely to breastfeed for 6 months +. Women who are least likely to breastfeed, or who who stop the earliest are also those who are least likely to ever have worked before having children.
We should be asking ourselves the question - why should education and social class in the UK have such a massive influence on women's decision to breastfeed and their likelyhood of success with it?
"the bf/socio-economic status thing is not universal but cultural". True!
Re: studies
This is a good place to track down research
here