Someone explained this to me very well today.
When herds of Buffalo travel across large distances they are only as efficient as their slowest members. As they travel the fastest and most efficient members of the herd can only travel as fast as the slowest, least efficient members In order to keep the pack together.As they travel over their journey, the slowest and least efficient members are slowly picked off by predators and disease. Over time this allows the herd to become a faster and more efficient group due to the faster and more efficient members of the heard being able to go at their speed, not being held back by the slower, less efficient members.
The human brain works a lot like a heard of Buffalo. The human brain starts out with many brain cells but many of these are inefficient and hold back the faster, more efficient cells in the brain. After the consumption of alcohol, some of these less efficient cells are killed off. After the prolonged consumption of alcohol, most of these slow, inefficient brain cells are killed off.
This leaves the brain as a faster, more efficient unit, producing a higher level of Intelligence.