Oh wow, that Listverse article linked to is terrible. Lots of discredited conspiracy theories (e.g. missing cosmonauts) and urban legends (e.g. Gloomy Sunday) presented as fact.
Though my favourite bit is that to support the "weight of the soul" conclusion — which they don't even summarize correctly; 21 grams wasn't an average, but a single result that he decided was the most accurate — the author links to a Snopes entry that has this to say at the end:
What to make of all this? MacDougall's results were flawed because the methodology used to harvest them was suspect, the sample size far too small, and the ability to measure changes in weight imprecise. For this reason, credence should not be given to the idea his experiments proved something, let alone that they measured the weight of the soul as 21 grams. His postulations on this topic are a curiosity, but nothing more.
(That said, the article from the Atlantic linked to is quite interesting.)