Transactive memory explains the concept of the knowledge that already exists around us externally and explicitly. I read about its discussion in Psychology of Everyday Things as a way to reduce errors made by users and lessen the burden of remembering things to do. This can be in the form of signages, labels, meters and so on.
We tend to remember very important things, like our home's or parent's telephone numbers and our I/C number.
For less important things, we deal with them a in slightly different way. We remember where we might find them instead. For example, we might not memorise the number of the contact number of our suppliers, but we do remember where we keep them. For that reason, we use notebooks, phone directories, business cards, software and other people.
In Wegner's experiment, he tested 59 dating college students couples who have been seeing each other for at least 3 months. They were then separated into two groups: those paired with their partners and those paired somebody else (people they don't know). The were asked to read 64 statements like "Midori is Japanese melon liqueur" and "Luke and Laura got married on 'General Hospital'". After 5 minutes, the pairs were asked the write down as many statements as they could remember.
Wegner found that pairs that know each other got significantly more statements right than the pair who don't know each each. He theorises that when people who are in a relationship for some time develops a joint memory system. This system only helps them to remember more things but also 'organises' the remembering process according who has the best understanding.
The idea of transactive memory may also not be exclusive to intimate, romantic relationship and probably observable in other relations like among colleagues in a team. This idea is still new and in need of further research to see, for example, how transactive memory takes place in a workgroup or department and in couples who have been married for a long time (a decade or longer).
Regarding the pain experienced by divorced people, Wegner writes:
Divorced people who suffer depression and complain of cognitive dysfunction may be expressing the loss of their external memory systems... They were once able to discuss their experiences to reach a shared understanding... They once could count on access to a wide range of storage in their partner, and this, too, is gone... The loss of transactive memory feels like losing a part of one's own mind.
(Picture from anyjazz65's flickr set, licensed under Creative Commons.)