Now, when you raise a kid it's easier to understand what's happening when she/he has the same kind of behaviors, physical type (including heredital illnesses) as you.
For instance if you have weak body defenses and your kids are too, you'll be pretty understanding and know first hand lots of ways to help them, and they'd be handicaped put in other hands. Your kid won't be some random living being alien to you [1], the outcome of putting a kid in another random family doesn't make a balanced experiment about success or failure in society.
[1] that happens with adoption for instance, but that doesn't make a neutral experiment either.
For instance if you have weak body defenses and your kids are too, you'll be pretty understanding and know first hand lots of ways to help them, and they'd be handicaped put in other hands. Your kid won't be some random living being alien to you [1], the outcome of putting a kid in another random family doesn't make a balanced experiment about success or failure in society.
[1] that happens with adoption for instance, but that doesn't make a neutral experiment either.