A little biological background is helpful. Semen (or seminal plasma) contains only 1 to 5 percent sperm. The rest of the chemical composition includes more than 50 compounds with various functions. A few of the more notable elements include cortisol (increases affection), estrone (mood elevator), prolactin (a natural antidepressant), oxytocin (mood elevator), thyrotropin-releasing hormone (antidepressant), melatonin (induces sleep), and serotonin (well-known antidepressant neurotransmitter), among many others. It's not a stretch for researchers to then consider whether semen would have antidepressive effects, given its composition.