Magic numbers: Society and what is "normal"

I was involved in a thread once upon a time and got trolled (expected if you talk about anything of significance -- and sometimes even if you are just talking about the weather). This was a thread talking about one of society's "magic numbers".

These are part of a group of numbers which we use based on statistical information. As Mark Twain once said (he said that the British Prime Minister Disraeli said it first): "There are three kinds of lies: lies, damned lies, and statistics."

For example, in current society a number called the Body Mass Index (BMI) is used to determine whether you are overweight, underweight, or in a "good range". This number is a ratio of height to weight and provides reasonable results for 80 to 85% of the population. For the other 15% to 20% of the population, the number just doesn't work well. If you are an athlete (or work out a lot and have more muscle) it doesn't work well. If you are a total couch potato with almost no muscle it will actually give you a better result than you "deserve". If you truly do have "thick bones" you will be at a disadvantage. This doesn't matter that much except if people are basing other things on that number -- insurance companies and computerized social services for example.

Why do we use this "magic number"? It is quick. It is easy. It is cheap compared to other, more accurate methods of body fat percentage calculations. However, even if one used a more accurate measure than that of the BMI, it would STILL not be accurate for everyone because there is no "one body shape fits all". For some, to hit that "ideal", one part of their body would have to be UNDER-weight in order for the average to be correct.

There are other "magic numbers" used. The "age of consent" is a magic number which, in more technical society, indicates that a child has become an adult. They are able to sign contracts, do things without permission from parents or guardians, get married and so forth. This magic number ranges from 13 in Japan to 21 in Bahrain. In some non-technical societies, it depends on the age when menarche sets in for women and "rites of passage" for men.

Many people get the direction of "age of consent" backwards -- thinking that a higher age of consent protects the child more from society. In reality, the family, church, or government usually have many options to do what they want with children at the age they think is right. The "age of consent" is what gives people the right to control, for themselves, decisions that affect their lives.

These numbers are set for two principal reasons. One is to prevent abuse of others by pushing them into activities (such as marriage or other things) before they can really make good decisions for themselves. The other is to determine an "age of emotional and mental maturity" which is taken to be an indication that they can make good decisions.

This type of magic number is determined almost solely by societal norms. It is usually lower in agricultural societies and higher in societies that require a longer period of education and social adjustment. It is also higher in societies where familial, and religious, control of women is greater. But, as is true with the BMI, it is (at best) a statistical reflection. Some will not be ready at age 30 to properly make decisions for themselves. Some might be ready at a very young age. There have been no psychological studies and are unlikely to be such.

Even the "age of consent" is not for everything. There are separate "magic numbers" for voting, being drafted for war, being able to drive, being able to work full-time, for purchasing and using legal drugs, and so forth. In many instances, the society would like to prohibit the activity but do not have sufficient backing from the populace to do such. Therefore, they set an age which most of society agrees is appropriate.

In all of these areas (and more), the "magic number" is sometimes determined by a statistical averaging and sometimes determined by societal norms. It is rare that the number is backed up by thorough, and consistent, studies -- which is why it is "magic".

What other "magic numbers" are you aware of within society? Do you know of any that have a researched background reason? Others that do not have such a background?