What are the genetic causes of the differences in behavior between European and Asian infants?

One cause may be the 7R allele of the D4 dopamine receptor gene, which is associated with Attention-Deficit/Hyperactivity Disorder (ADHD) and is very rare in East Asians. Nonetheless, as with differences in intellectual capacity, we're probably looking at an accumulation of small effects at many different genes. Natural selection acts on what genes produce, and not directly on genes, so there is no reason to believe that a single behavioral outcome has a single genetic cause.