Statistics can't help

As we've been discussing nature vs. nurture in class and whether statistics are reasonable evidence for either side.  The nature side of the debate has more of a advantage because their argument is behind science.  With science, statistics dealing with matters such as brain structure, are more reliable.  However, on the nurture side, statistics aren't as reliable.  How can we actually 100% determine if a parent's guidance effects our development? How can we distinguish between environmental causes and parental?