Behavioral design means influencing what people do (behavior) by modifying (designing) the environment in which the behavior takes place.
The changes in context (aka. behavioral interventions) can have different levels of firmness - the extent to which they restrict the users' freedom exhibiting behaviors that are different from the one desired by the designers.
At the first level of firmness, the changes in the environment are subtle and aim at guiding behavior.
For example, the do not disturb door-handle tag in hotels is a relatively subtle cue of what the desired behavior is – do not come into the room (to clean it). Even if the instruction is clear – do not disturb, the tag doesn’t prevent the person to open the door.
By the way, I’m sure people ask for privacy because they are watching TED Talks. After all, it would be embarrassing to have your privacy violated while you are watching TED Talks.
At the second level of firmness, the behavioral design features are much stricter in guiding behavior. They make it difficult, but not impossible, for the undesired behavior to occur.
Take the example of this do not disturb sign that I found on a cruise ship.
It may seem the same as the door handle tag, but it actually is very different and not because it doesn’t involve TED Talks.
The Big difference is in the fact that this sign is not hanging from the handle and it obstructs the magnetic key-card slot. To the untrained eye, it may seem like a subtle difference, but it’s a big one because, in order to open the door, one has to remove the sign form the key slot and purposefully and consciously ignore / disregard the instruction on the sign.
This obstructive design still allows for the undesired behavior to occur (removing the sign is not a complex or difficult operation), but it makes the violator much more aware that she is doing something she shouldn’t.
In other words: in the case of the obstructive sign, in order to do the wrong thing, one needs to have a much more personal and direct involvement than in the case of the door-handle tag which allows for the door to be opened without needing to be removed.
At the third level of firmness, the changes in the environment force the desired behavior. In a nutshell, you can’t do the behavior you want to do without doing the desired behavior first.
Take the example of this toilet that can be flushed only if the lid is put down.
Judging just by the firmness of the design, I guess that putting the lid down and having the toilet covered when not in use are highly desirable.
The person can flush the toilet only after putting down the lid. Quite interestingly, once the lid is down and the toiled is flushed, there is no immediate reason to lift the lid, thus the toilet is always covered when it is not used.
I don’t know exactly why it is important to close the lid while flushing, but the best explanation I have is that by keeping the lid closed the risk of objects falling accidentally into the toilet is decreased, which helps with not having clogged toilets on a sea vessel that can be at sea for several consecutive days.
This design approach in which one behavior can be performed only after another behavior has been performed previously is used in ATMs as well. When you withdraw cash, the ATM first releases your bankcard and only after you removed it from the machine it releases the cash.
This design feature was introduced because, in the early days of ATMs, people got the cash and left their bankcards behind, which created a lot of nuisance for both banks and customers.
How firm should the design be?
It depends on (1) the risk (both the probability and impact - damage) of the undesired behavior occurring and (2) the level of inconvenience to the user of performing the desired behavior.
Most times subtle interventions are enough, but sometimes firmer ones are needed.
In the case of ATMs, on the one hand, the inconvenience for the client to first remove the bankcard and only after to get the cash is very low. On the other hand, the damage of forgetting the bankcard in the machine is high as it leads to increased costs for the bank and discomfort to the client.
In the case of the toilet that requires the lid to be down in order to flush, the inconvenience for the user is small and the damage of having clogged toilets on a cruise ship is large.