You had to consult relatives and ask them: "do you know if the deceased was opposed to this"? This meant the way to opt out was to tell your relatives. It has been the case for 40 years. [1]

There were posters in most hospital waiting rooms telling people to do this (which I found creepy as I found myself there a few months ago, waiting for news on whether a relative would live or die, by the way). See [2] for an example.

Now the only way to opt out is to send snail mail (!) to be added to a nationwide file. It should become possible to do it over the Internet someday.

Fun fact - we have a pin card that has been designed to store that information since 2006, but the feature has never been made accessible to the public. [3]

[1] (fr) http://www.lemonde.fr/les-decodeurs/article/2016/12/22/don-d...

[2] (fr) http://fr.slideshare.net/amfe/guide-du-don-dorganes-2015-de-...

[3] (fr) https://www.agence-biomedecine.fr/Carte-Vitale-2-et-don-d-or...

Oh, so instead of saying "I don't know" to trigger the default, people were possibly lying and taking a decision on behalf of their deceased relative…

Yes, I added a source (the first one) which confirms that. 32.5 % of the time the relatives said no, but polls showed that it should only have been 21 %.

Lying is a rather strong word, perhaps they thought that their relative was inclined to refuse donation. And default has no meaning here because they know that their answer decides the outcome; letting it fall to a “default” option decided by others seems to be rather irresponsible to the deceased relative.

Death is a rather traumatic event. Many people won't think straight in this situation. Some may just blurt out "no" because they're in no state to respond to any kind of request.