Here some pretty interesting info:
http://people.howstuffworks.com/tipping1.htm
The Psychology of Tipping
The most prominent reason for tipping appears to be more of a guilt issue than a gratitude issue. We know that tipping, at least in the United States, is expected, and if we don't tip, we stand the chance of angering the server, and then who knows what our next visit to that restaurant, airport or salon will be like?
Also, our assumption that tipping is designed to encourage good service for our next visit may or may not be accurate. What if we are at a restaurant that we know we won't be back to? What then is our motivation to tip? Some argue that it is a way of making ourselves feel better about being served because we know the waiter works hard and isn't paid well.
Can the Weather Determine Tip Amounts? *
Temple University psychologist Bruce Rind found that weather can be influence tip amounts. Rind conducted a study in a room with no visual clues to the weather. "When the server told guests that it was raining, tips averaged 19 percent of the bill. But describing sunny skies sent the gratuity rate soaring to 24 percent" [Source: Pyschology Today].
Believe it or not, a lot of research has gone into why we tip and what makes us tip more or less for similar service. Research has shown that the quality of the service we receive isn't always reflected in the tip we leave. Many who have studied the practice have discovered that excellent service only draws a marginally higher tip than average service. Other things the server might do, however, do make more of a difference -- probably without our even realizing it.
Cornell University's Center for Hospitality Research conducted several studies revealing some other interesting facts about server habits that can boost tip percentages. Here are a few of them:
Briefly touching patrons' shoulders may increase tip amounts.
Touching - Waiters experienced a tip increase from 11.8 percent to 14.8 percent of the check total when they briefly touched the shoulder of the customer. Both men and women left higher tips when touched, and although younger customers increased their tip amount more, all ages increased the tip by some amount [Source: Center for Hospitality Research at Cornell].
*Squatting - Two studies showed that waiters who squatted next to the table when taking orders and talking with customers increased their tips from 14.9 percent of the bill to 17.5 percent of the bill in one study, and from 12 percent to 15 percent in another study. Apparently, the eye contact and closer interaction creates a more intimate connection and makes us want to give the server more money [Source: Center for Hospitality Research at Cornell].
Including candy with the check may increase tip amount.
Giving candy - A study that involved giving customers a piece of candy with their bill showed an increase in tip percentage from 15.1 percent to 17.8 percent. Another study in which servers gave each customer two pieces of candy with the bill increased the tip from 19 percent to 21.6 percent of the bill. Still another study showed that the way the server gave the customer the candy had the largest impact on the increase of the tip: This study had the server initially give each member of the customer's party one piece of candy and then "spontaneously" offer a second piece of candy. This method increased the tip to 23 percent of the bill [Source: Center for Hospitality Research at Cornell].
. I think if I was to hand them a cash tip they would think I am crazy
You honestly do NOT know what you are talking about! Please just stop while your ahead and say something again that isnt true. 
I don't like linking it to the key and carrying cash can be a pain.
