How can it be possible to distinguish between that which is real, and that which we think that is real? When is something only real in someones head, and when is something considered to be a reality? Of course science can provide us with established standards by which 'things' can be measured, but most of the time of our lives we spend OUTSIDE laboratories. So much for a scientific approach.
A rational approach would be the next best thing. If one person has some ideas you would consider humbug, you might label that person delusional. However, to the person in question, his ideas are 'real', and this will be reflected in the way he will interact with what both of you would call 'reality'. Should he believe you to be a monster, or that you are an embodiment of all that is evil, you might say that he's insane, or that he's living in a fantasy-world, but does that make it less real when he comes round one night and kills you in your sleep? Even if the person in this example is considered to be mentally ill by 100% of all other people on planet Earth, including yourself, whatever he believed in was 'real' enough for you to get actually killed.
Another good example here would be 'God'. It doesn't matter how skeptical you are, but you cannot deny that the influence of 'God' can be clearly observed as actions of people in the physical world. Wether this is because he really exists, or wether it's just because such an incredible amount of people believes Him to exist has nothing to do with this. Fantasy or not, 'God' is real enough to cause effects in the way people behave. And no matter how much you might believe that all such people are suffering from some kind of mass-hallucination, or whatever, doesn't change one bit. As long as 'God' is considered to be real by so many people, 'His' existence is a reality, regardless of wether the entity itself is merely made up out of human fantasy and imagination.
No matter where on Earth you may look, societies mainly consist of a number of beliefs that are generally accepted as 'reality'. This is not only true for beliefs (or assumptions) concerning what is right and what is wrong, but also applies to material stuff, like money. If an entire society believes in the value of otherwise completely worthless pieces of paper (or completely meaningless digital numbers, in the case of modern society) and it is naturally assumed that without money you're nowhere, this provides a drive for people to acquire as much of this stuff as possible. People may even place the acquisition of it before other, ethical ideas. This is clearly the case is most of the Western world.
The bottomline is, that whatever people believe, becomes part of their reality. If two or more people believe the same thing, this reality becomes a collective truth. In this case they all believe the same thing, and each individual is likely to experience the discovery that the others happen to share his/her beliefs as a confirmation. Even if the belief seems downright absurd to an outsider, the outsider must realize that these people will act according to this belief, wether this seems reasonable or not.