It is well known that people who aren’t computer security experts tend to ignore expert advice on computer security, and (to some extent as a consequence) get exploited. This paper is not the first, or the last, to investigate why; see also the
What Deters Jane papers  ,
So Long and No Thanks for the Externalities, and
Users Are Not the Enemy. However, this paper provides a much more compelling explanation than anything earlier (that I have read), and a lens through which to view everything since. It’s plainly written and requires almost no specialized background knowledge; you should just go ahead and read the whole thing.
For those not in the mood to read the whole thing right now, I will summarize. Wash conducted
semi-structured, qualitative interviews of 33 home computer users, who were selected to maximize sample diversity, and specifically to exclude security experts. From these, he extracted a number of what he calls
folk models—qualitative, brief descriptions of how these people understand various threats to their computer security. The term
folk is used to describe a model which accurately reflects users’ understanding of a system, and is broadly shared among a user population, but might not accurately reflect the true behavior of that system.  In this case, that means the models reflect what the interviewees think are the threats to their home computers, even if those aren’t accurate. Indeed, it is precisely where the model is inaccurate to the real threats that it provides an explanation of the phenomenon (i.e. users not bothering to follow well-publicized security advice).
A key aspect of all the models presented is a division of security threats into
Virus is used by the interviewees as an umbrella term, corresponding most closely to what experts call
malware—any piece of software which is unwanted and has harmful effects. (One model expands this category even further, to include programs which are unintentionally harmful, i.e. they have bugs.) The models differ primarily in users’ understanding of how
viruses get into the computer, and what they are programmed to do once there. This can be very vague (e.g.
viruses are bad software you don’t want on your computer) or quite specific (e.g.
viruses are deliberately programmed by hackers as an act of vandalism; they cause annoying problems with the computer; you get them passively by visiting sketchy websites—an expert will acknowledge this as true-but-incomplete).
Hackers on the other hand are people who are actively seeking to exploit computers; most interviewees share the understanding that this involves
taking control of a computer remotely, thus allowing it to be manipulated as if the hacker were physically present at its console. (Many of them hedge that they do not know how this is done, but they are sure that it is possible.) The models here differ primarily in the motives ascribed to the hackers, which are: vandalism, identity theft, or targeted identity theft and financial fraud. This last is one of the most telling observations in the entire paper: a significant number of people believe that they are safe from hackers because they have nothing worth stealing or exposing. (Again, an expert would recognize this as true-but-incomplete: there really is a subpopulation of black-hat actors who specialize in going after the
big fish. The catch, of course, is that the data exfiltrated from a
big fish might include millions of people’s personal credit card numbers…)
Having presented these models, Wash runs down a list of standard items of home computer security advice (drawn from Microsoft, CERT, and US-CERT’s guides on the topic) and points out how many of them are either useless or unimportant according to these models: for instance, if you think you can’t get viruses without actively downloading software, then antivirus software is pointless, and patching only valuable if it eliminates bugs you trip over yourself; if you think hackers rarely, if ever, vandalize a computer, then backups are not necessary to protect against that risk. He closes by comparing the novel-at-the-time threat of botnets to all the models, observing that none of them account for the possibility that an attacker might subvert computers indiscriminately and automatically, then use them only for their Internet connection. In particular, all of the
hacker models assume that computers are attacked in order to do something to that computer, rather than as a means to an unrelated goal (sending spam, enlarging the botnet, executing DDoS attacks, …) and that the hacker must be doing something manually at the time of the attack.
The landscape of security threats has changed quite a bit since this paper was published. I would be curious to know whether ransomware, RATs, third-party data breaches, and so on have penetrated the public consciousness enough to change any of the models. I’d also like to know whether and how much people’s understanding of the threats to a mobile phone is different. And, although Wash did make an effort to cover a broad variety of non-expert home computer users, they are all from the general population near his Midwestern university, hence mostly WEIRDos. I’m not aware of any studies of this type conducted anywhere but North America and Europe, but I bet it’s not quite the same elsewhere…