A new survey claims today that mothers are "more critical of their daughters than their sons and let boys get away with more". I can only comment from the perspective of having been on the receiving end of this scenario, and in my case I'd say it's rubbish - from what I remember, my mother treated my sister and me pretty much equally from the point of view of getting away with anything, and I certainly don't remember her being unduly critical of my sister.
Having said that, I'd be the first to admit I was a "mum's boy". My mother and I were very much alike temperamentally: I was always given to a 'what you see is what you get' personality, whereas my sister was given more to quiet brooding and occasional sulking and in that respect took after my father. The other thing that needs to be said is that she's seven years older than me, and she doubtless compared the treatment and discipline meted out to me compared to a perhaps stricter standard which had prevailed seven years earlier. Whereas I in turn got packed off to bed earlier, and got less pocket money... always accompanied by a 'when you're her age' line of reasoning.
I did better at school than she did, putting the lie to the modern educational view which regards boys as "under-achievers" compared to girls, and got rewarded for my efforts, but neither of us got spoiled. Looking back on it all, I think it's true to say that in some ways we were treated differently, but that was as much to do with us as individuals, and how and where we grew up, than either of us being favoured over the other.