Dopamine/Serotonin

Effects of dopaminergic and serotonergic drugs (e.g. cocaine and MDMA respectively) suggest dopaminergic pathways underlie egoistic processing while serotonergic pathways underlie altruistic processing. Emotional responses to personal triumphs/defeats are mediated by dopamine. Emotions such as empathy and affection are likely mediated by serotonin. Addictive behaviors are known to involve dopaminergic pathways. This might account for MDMA's being less addictive than cocaine.

SSRIs e.g. lexapro, prozac, might affect depression by enhancing thoughts/feelings of intimacy.
Dopaminergic antidepressants (e.g. bupropion) likely affect depression more directly by enhancing egosyntonic thoughts/feelings.

Hypothesis: species with altruistic behavior (e.g. mothering) will have serotonergic pathways, those without will not. e.g. Bee's do not have serotonergic pathways (they do have dopaminergic pathways). Chickens (and other species with mothering behaviors) do have serotonergic pathways.