Final answer:
The substance John inhaled that increased empathy and trust is likely oxytocin, a hormone closely associated with social behaviors like bonding and trust. It is not melatonin, endorphins, or arsenic, which have different effects on the body.
Step-by-step explanation:
After inhaling a secret substance, John displays more empathy and is more trusting of others. It is likely that the secret substance contained oxytocin. Oxytocin is a hormone known to play a significant role in social bonding, sexual reproduction, childbirth, and periods following childbirth. This hormone is often referred to as the 'love hormone' because it increases behaviors that include bonding, empathy, and trust between individuals.
Melatonin, while it is involved with the body's circadian rhythms and regulates sleep, does not have a direct impact on empathy or trust. Endorphins are natural painkillers produced by the body and induce feelings of pleasure or euphoria, not necessarily trust or empathy. Lastly, arsenic is a toxic substance and would not increase trust or empathy, but rather would be harmful upon inhalation.