Your genes decide your protein and formone factories - whether they produce more or less serotonin, dopamine, noradrenalin, testosterone or a million other things that need to be in balance for the size of body they are in.

In very few of us are these all in balance - all the time. But the basic production process can be influenced by both internal and external causes - which is where nurture comes in.

You can't be me, I'm taken