In ambiguous situations, some people might view an act as unintentional, while others see it as intentional. Your sister helps clear the dishes after dinner and drops a cherished serving dish you brought back from a foreign country, shattering it into a dozen shards. Everyone else accepts her apology for the accident. You're sure she did it on purpose to get back at you for some slight she may have felt during the course of the mealtime conversation.
Most adults who have learned the basics of science are likely to see processes in the natural world--such as thunderstorms, earthquakes, or the eruption of volcanoes--in mechanistic terms. We don't think any intentional agents bring about such events. We learned in school, however, that many of our ancient ancestors perceived the natural world as full of "spirits" or invisible intentional agents. It seems likely that intentionality bias emerged with the evolution of the earliest humans. Several studies on intentionality bias in children indicate that a natural way of perceiving and making sense out of the natural world is to see intentional agents behind the movements of many things that adult scientists attribute solely to mechanistic forces. Intentionality bias in children has led Justin L. Barrett, a psychologist at Fuller Theological Seminary, to claim that we are "born believers" in religious claims and that "religion comes nearly as naturally to us as language." (For more on Barrett and his book Born Believers, see The Daily Beast.)
I would argue that all that we're justified in inferring from the natural bias toward perceiving intentional agents as behind the movements of both animate and inanimate objects is that it is natural to think anthropomorphically about natural events and that it's natural to think that others like us have intentions like we do. I think it is a long way from the intentionality bias to religion of any kind, though clearly some religions have taken advantage of this bias in promoting their beliefs. I don't think it is inevitable, though, that seeing agents behind weather patterns or geologic events must lead to personifying those agents into beings like ourselves, only better. Attibuting good looks, immortality, perfect health, and magical powers to gods may have been the next step for many early human societies, but was it inevitable that that step be taken? Once that step was taken, was it inevitable that humans would start trying to control these agents with bribes of virgins, burnt meat, and sizzling rice soup (ok, I made up the latter, but different cultures offered up different things to their gods based on local tastes). I don't think so, nor do I think it was inevitable that monotheism or belief in an agent who is bodiless but powerful enough to create the universe by an act of will would emerge and supplant polytheism. There are some logical gaps in moving from seeing intentions in a thunderstorm to seeing a "pure spirit" whose intentions created the entire universe and to building worship houses to appease and honor this supreme being. I think it is a long way from seeing purpose in an earthquake to seeing earthquakes as direct acts by a supreme being to harm creatures that exist only because the being wills it. There is no necessary connection between seeing agents everywhere in nature and seeing everything in nature and human society occurring only because some invisible being with extreme powers wills it. Also, the leap to making claims that some of these invisible agents communicate with humans in dreams or visions, revealing the gods' intentions, wasn't inevitable. I would not say that intentionality bias has led inevitably to claims that one's life and ambitions are part of some god's plan. But without belief in intentionality, no gods would likely have been created by humans. (For those who like their logic straight, intentionality is a necessary but not a sufficient condition for belief in gods.)
Clearly, the intentionality bias is stronger in some people than in others. Combined with the human need for significance and meaning, we have at one extreme people who see everything as purposive. Nothing happens by accident. Even accidents have a meaning. An invisible supreme being not only watches over everything that happens, the being doesn't let anything happen except according to plan. At the other extreme from those who think somebody's in control of everything are those who see no intentionality at all in the natural world and who see all human acts as determined by causes, even intentional acts. Many people see no need for gods, spirits, or any other invisible intentional agents to explain the natural world. In the social world, there seem to be some people who find it very difficult to "read the minds" of other humans, but who have little difficulty in "reading the minds" of cattle or other animals. Temple Grandin, who was diagnosed as autistic as a child and who says that today she would be diagnosed with Asperger syndrome, comes to mind. She attributes at least part of this ability to empathize more with cattle than with humans to the fact that she thinks in pictures. There has been some research that suggests a deficit in the intentionality bias is related to autism and Asperger syndrome.
The tendency of early humans and modern children to see intentional agents behind mechanistic processes may be an expression of an essential adaptation for social creatures such as humans. The inability to perceive the intentions of others is a major hindrance to social development. It is obvious that the intentionality bias is a necessary condition for perceiving the intentions of others. If you can't perceive intentionality, you certainly can't perceive the specific intentions of others.
The advantages of intentionality bias to social beings are many. Cooperative collective action may be fine for ants and termites functioning mechanistically and without conscious regard for the intentions of their comrades in building or carrying supplies. But the scope of their actions is severely limited by the inability to perceive purposiveness in each other's behaviors. Instinctive reactions to others, unaided by a natural tendency to perceive their intentions, would have kept our species from evolving into the creatures that built pyramids, aqueducts, skyscrapers, and, yes, cathedrals. The inability to determine whether another animal's intentions are benign or malevolent would be a great disadvantage to any mammal.
Some have linked mirror neurons to the intentionality bias. Mirror neurons discharge both when a person (or monkey) executes a motor act and when it observes another individual (a human being or another monkey) performing the same or a similar motor act. The following abstract might help clarify the idea that mirror neurons are linked to intentionality bias:
Our social life rests to a large extent on our ability to understand the intentions of others. What are the bases of this ability? A very influential view is that we understand the intentions of others because we are able to represent them as having mental states. Without this meta-representational (mind-reading) ability their behavior would be meaningless to us. Over the past few years this view has been challenged by neurophysiological findings and, in particular, by the discovery of mirror neurons. The functional properties of these neurons indicate that intentional understanding is based primarily on a mechanism that directly matches the sensory representation of the observed actions with one's own motor representation of those same actions. These findings reveal how deeply motor and intentional components of action are intertwined, suggesting that both can be fully comprehended only starting from a motor approach to intentionality.
The tendency to infer intentionality in the behavior of others has been the subject of much study. There have been several experiments with both children and adults that have shown that both have little difficulty in seeing the movement of computer generated colored shapes as intentional. I see several benefits stemming from such research. One is to determine how strong the intentional bias is in people; the other is to try to help those who suffer socially because they do not have a strong intentional bias. Another benefit is that such research helps establish just how natural and strong the tendency to find causal relationships, even misguided intentional relationships, is in human beings. Intuitively, an adult might see one triangle as "chasing" a circle, but on reflection most adults recognize that chasing requires intentionality and triangles aren't intentional agents. Intuitively, one might perceive invisible agents guiding natural processes or watching over what happens to you, but on reflection adults should recognize the implausibility of these agents actually existing. After all, if you found yourself unable to recognize intentionality in others, the last place you'd think to look to rectify this problem would be for intentional agents controlling the situation.
If humans had not abandoned the belief that everything in nature is intentional--sometimes called the teleological view of the universe--science could not have developed. Well, it could have developed if we posited some sort of Leibnizian universe where the mechanistic and the teleological were synchronized by a supreme agent in control of everything. I'll leave it to others to try to explain why Leibniz's attempt to harmonize materialism and idealism, mechanism and telology, and mind and body didn't catch on. Suffice it to say that his ideas are a bit farfetched and rather implausible. In any case, if everything in nature is an effect of some intentional agent, order and predictability becomes a problem. Some might think this problem disappears if we posit a single agent for everything, which might work provided the agent isn't whimsical. In any case, as others have noted: such an agent is an unnecessary hypothesis for science.
It may be of interest to those trying to connect intentionality bias with religious belief that psychology, which began as the study of the mind or psyche, became widely accepted as a science when it became the study of behavior. Why? Because minds seem hidden from observation whereas behavior is measurable and observable. With the development of neuroscience, the invisible mind has been shelved in favor of the brain, whose functions can be made visible by various scanning devices.
So, I wouldn't claim that intentionality bias explains why belief in gods is popular or why religions evolved into the major social institutions they've become. But the appeal to intentional agents behind natural events is enhanced for many people because of this bias. The appeal of gods and religions, however, must involve other things besides being in harmony with our natural instinct to see movements in animate and inanimate objects as being caused by the will of intentional agents. I'll leave it to others, however, to address such issues as the desire for immortality, justice, protection, and meaning. There is one topic, however, that I won't leave to others to discuss: the connection, if any, between intentionality bias and the belief that plants, animals, planets, and even the whole universe is designed and purposive. Of course, if it is natural to believe in design, it is natural to believe in a designer.
Some researchers have found evidence that children naturally perceive the world as designed. It should not be surprising that evolution by natural selection is not intuitive, since the idea didn't emerge until the middle of the nineteenth century. So, it should not be surprising that education in science would be a necessary precursor to giving up the idea of a designed universe in favor of one that looks like it's designed but is actually the result of natural forces playing out over billions of years. It may be intuitive to perceive order as coming about from intentional agents, while disorder can be perceived as coming about with or without the intervention of intentional agents. But in addition to our natural, instinctive way of seeing the world, we also have the ability to reflect on what we observe and overcome our natural instincts. Just because it is natural to see the world as designed doesn't mean the world is designed. We need not be a slave to the brain, which, after all, deceives us about many things. Why should we be surprised if the brain tricks us into believing in gods and other intentional agents as the designers of plants, animals, and the vast expanse we call our universe?