Most infants aren't tested at birth for such things. They are tested a few weeks after birth. If they were born to anxious moms who hold them, change them, feed them and play with them - anxiously - perhaps they are absorbing mom's anxiety rather than being born with their own.
Having been raised by a very anxious mom and watching mothers with PTSD raise their children, I have to wonder if there isn't an environmental factor, especially when all of a mother's children seem to have anxiety. That is statistically unlikely for a biological factor. An article on BBC News quotes research which has shown a mother's anxiety during pregnancy can double her risk of having a hyperactive child. Now this is curious. What does anxiety have to do with hyperactivity? I have long believed that what is diagnosed as "hyperactivity" in children is actually anxiety.