Parents repeatedly hear that they should never compare their children to each other. As a pediatrician with 30 years of experience, I think this is one of the most ridiculous things I have ever heard. Not only do we compare our children to each other, but we also compare them to other people's children. Why? Because it's impossible not to.
It's instinctive for humans to compare things. We compare which apples to buy at the grocery store. We compare which clothes to wear to work. We compare which online newspapers to read.
So where does this lopsided wisdom come from? I suspect it's derived from not understanding the difference between comparing and judging. Things can be different without one being superior to the other. When it comes to children, there is not just one trait you are comparing. People are more complicated than apples or clothes, and most parents appreciate different attributes in each of their children. The flip side of this is also true. Namely, each child can frustrate us in different ways.
When this topic comes up in my office, I start by telling parents what I just said. I then emphasize that parents should be careful not to compare their children unconsciously, i.e., without being aware of it. It's also perilous to censure one child by invoking the more desired traits of his or her sibling. The surest way to invoke a child's anger (and passive-aggressive behavior) is to tell him to act more like his sister. In the interest of full disclosure, I have done this myself on more than one occasion. However, when I do it, I realize that I've blundered.
I have two teenagers who are very different. I realize it might sound stereotypical, but I enjoy sports and go to movies with my son, whereas I discuss books and social issues with my daughter. I am proud of both of them, but in different ways. In addition, they've each done things I have not been thrilled about.
Comparing your children to your friend's kids is trickier because you never have all the facts regarding someone else's family. You may still find that you like certain aspects of your friend's children more than your own. That's okay. Just keep it to yourself and remember that most kids behave better with people other than their own parents. Ironically, if your children like certain aspects of their friend's parents better than you, they won't hesitate to tell you about it!
Finally, it's important to remember that children have big ears. They love to eavesdrop on their parents because it's exciting to hear what adults have to say when they are alone. If my son heard me compare him to his sister, I am setting myself up for trouble even if it was a fairly innocent comparison. I can think these things, and I can discuss them with my wife. But this should only be done when all children are accounted for.