The prevailing theory is that sexual reproduction, which requires two genders, allows genetic intermingling that overall is very good for both the individuals and the species as a whole. Asexual reproduction, on the other hand, is largely static, where each offspring is genetically identical to every other.
The decrease of biodiversity, messing up the ecosystem in regards to the food web and chain etc.
Neurotransmitters that have narcotic-like effects are called endorphins. These neurotransmitters are responsible for our feelings of excitement or satisfaction.
