Psychologist George Miller found 2.5 bits to be an approximate limit for a certain type of channel capacity. In Watership Down, Richard Adams coined the term "hrair," referring to a maximum countable number, which for rabbits was four. IT guru Ed Yourdon defined hrair for a computer programmer at between five and nine.
Each of these experts attempts to quantify the optimal number of relationships.
The best explanation of why most humans have two or three children may lie in what Physicist Mitchell Feigenbaum discovered: chaotic behavior tends to occur when a certain parameter, called the first Feigenbaum constant, is approximately four and two-thirds, about the average human family size.
Could it be that the optimal number of relationships occurs when chaos is maximized? How could we ever make order out of that?