I'm looking for a scientific explanation as to why vampires would be incapable of entering a human's house uninvited.

There have been many terrific scientific explanations for some of the most interesting vampiric traits over the years, but this is one that I'm unable to find (Google, Stack Exchange). In my world there's the beginning of a vampire society, and some vampire resistance to sunlight, so I'm interested in other ways of hindering these otherwise super-powerful blood-suckers.

In a bunch of classical stories, vampires do have this limitation that prevents them from entering a decent folk's house without an express invitation. They hang out in the doorway, looking all innocent and politely asking if they may come in. If the good but foolish virgin/elderly/child says "Do come in, what can possibly go wrong" - vamp goes in, blood comes out.

This limitation is usually attributed to a measure of holiness (sanctity of a place of residence) vs. the vampire's unholiness, or a magical physical prevention trick. In one short story, I recall that the vampire was unable to enter a shabby hovel before being invited, even when pushed into the entrance by a powerful ogre.

But I'm looking for a scientifically (can be soft science) explained vampire, such as in these neat questions and answers:

How much blood does a vampire need?

Plausible explanation for lack of reflection in a mirror.

I was thinking about a psychological disability, but psychology is shifty and can be changed. Then I thought of a selective force field, but those can be hacked/altered. I really would like to have a physical, improbable to alter reason for a vampire to grind their teeth and be unable to take an extra step into my residence.

(enter virgin/elderly/child voice here:) Help! Scientific heeelp!

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    $\begingroup$ The problem with any physical force that prevents the vampire from entering is this: How does it suddenly stop working when some person speaks a few words? If Divine Power is out, Psychology is your best bet. $\endgroup$
    – Cyrus
    Commented Apr 12, 2016 at 7:17
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    $\begingroup$ I'm looking for a scientific explanation - perhaps you should rephrase as a science fiction explanation, since the iterative process of observation and analysis is not possible for fictional settings. $\endgroup$ Commented Apr 12, 2016 at 13:02
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    $\begingroup$ I always thought it was a CLASS thing - as if breaking and entering was beneath them. Vampires = classy, zombies = gauche. $\endgroup$ Commented Apr 12, 2016 at 20:00
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    $\begingroup$ This answer worldbuilding.stackexchange.com/a/7119/592 from one of the linked questions also covers this question. $\endgroup$ Commented Apr 12, 2016 at 20:32
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    $\begingroup$ My fiancee, her father and grandmother are vampires. I can test every science based idea you get, no problem :) $\endgroup$ Commented Apr 15, 2016 at 13:47

19 Answers 19


I think you can approach it from three different avenues, depending on the tone of your setting:

Psychological: Perhaps the root cause of vampirism in this world (such as a virus), inflicts an extreme form of OCD, hardwired into the vampire's brain. The vampire would be physically capable of entering someone's home, but they'd have an extreme, irrational aversion to doing so without an invitation.

The hardwired OCD could be a side-effect of one of the benefits of vampirism: ie, the same neurological change which allows superhuman senses/reflexes could cause this specific type of extreme OCD.

Physical: If you need the barrier to be physical, then I think the best bet would be a form of psychic influence. Similar to the psychological option, this could be explained as an unfortunate side effect of one of the vampire's powers.

For example, if the vampires in this world have the ability to perform mild mind-control, then their ability to "transmit" thoughts could also leave them vulnerable to the uncontrolled psychic emanations of humans.

A human in their own home would have a sense of psychic safety and confidence in their surroundings-- this causes a strong psychic field which prevents the vampire from breaching the boundary of the home.

When the human invites the vampire, they weaken their psychic defenses and allow the vampire to approach.

The benefit of this method is that it could also explain the effect of holy symbolism on vampires: someone with sufficient faith would essentially have a powerful psychic field around themselves at all times. This field could cause severe injury to a vampire who got too close.

Legal: I think this would be the weakest option, but it may fit in with the nascent vampire society mentioned in the original question. Vampires would have no physical block against entering a human's home, but the law of vampire society may strictly forbid such trespass (possibly due to an ancient treaty with humans?).

In this case, any vampire who violated the law would be viewed as the lowest form of criminal by their peers, and subject to execution, and possibly other harsh punishments beforehand.

This option would work best if the vampires in this world do not need to kill to feed, so any vampire who breaks into a human's home to attack them would be considered a criminal to begin with.

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    $\begingroup$ The "OCD" answer fits with a few other traditional vampire traits, like the compulsion to stop pick up (or count) spilled grain. $\endgroup$
    – mattdm
    Commented Apr 12, 2016 at 12:29
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    $\begingroup$ @Mattdm Wow, The Count from Sesame Street works on so many levels now that I know that. $\endgroup$ Commented Apr 12, 2016 at 16:26
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    $\begingroup$ I always wondered about the effect of crosses on a vampire. I mean, if we assume that vampirism can be communicated via blood - then any person could be made into a vampire, regardless of their religion. Would a Jewish vampire have a similar aversion to a Star of David? A Muslim vampire with a crescent and star, etc.? Or is it only the faith of the person wielding the holy symbol that counts? Does that mean atheists are inherently more vulnerable? Or can they wield a copy of Darwin's "Origin of Species" or something instead? Do you even need the symbol, or is faith alone enough? $\endgroup$ Commented Apr 12, 2016 at 16:55
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    $\begingroup$ @DarrelHoffman This again can potentially be explained by the OCD idea. Based on my knowledge of OCD, having strong religious feelings can play into OCD compulsions pretty strongly. For example, if vampires are themselves highly religious or anti-religious, perhaps doing something that they feel desecrates or venerates a religious symbol could trigger their OCD. $\endgroup$
    – Kevin
    Commented Apr 14, 2016 at 20:07
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    $\begingroup$ @DarrelHoffman Obviously, it depends on the particular mythology in your story. In a Doctor Who 7th Doctor story, The Curse of Fenric, the vampires are affected by the faith of the person they are facing. Some Russian soldiers use their (misplaced!) faith in the Soviet Union to repel some vampires, while a priest who had lost his faith in God was not saved, even though he was holding a cross against the vampires. $\endgroup$
    – CJ Dennis
    Commented Apr 16, 2016 at 6:38

In the novel Blindsight by Peter Watts, vampires have a neurological "glitch" that sends them into an epileptic fit whenever they see a right angle. Presumably the wiring of their visual cortex gives them a very unusual power of perception, but comes with a price. This is called the "crucifix glitch" and the idea is that in nature perfect right angles are very rare, so this glitch wasn't selected against.

I thought this was a brilliant explanation of the crucifix aversion, but you could also utilise it for the invitation problem. After all doors and windows have right angles, so a vampire with a crucifix glitch couldn't pass them with his eyes open. The only way a vampire could enter a house would be, if he was lead by the hand or directed by voice, with his eyes closed (Or possibly with his eyes open, but his vision switched off …).

So basically when our nasty bloodsucker comes into a little village, he has to switch off his vision, right angles everywhere, and proceed by hearing alone. Now he can do this a lot better than a blind human, in fact you wouldn't easily notice that he can't see, but he is still severely handicapped. It wouldn't do to be caught creeping around, touching walls searching for a way in or attacking humans in the open. His only shot at getting a pint of blood, is to be invited into the safety of a house by somebody he can overwhelm even blindly without making much noise.

Enter "the good but foolish virgin/elderly/child" seeing the traveler walking down the middle of the road, looking all forlorn …

So basically the invitation is necessary for two reasons: The vampire needs to hear a voice that leads him through the door without appearing blind (people are incredibly suspicious of the blind in these parts). And people have to let him into the house without making a fuzz, because without his vision he would have a hard time to escape from the horror of right angles that is a village. Only once he is behind closed doors with the "good but foolish" can he strike.

Edit: Did you ever wonder why they build houses like this in the old days? Now you know. OldHouse

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ Commented Apr 14, 2016 at 1:52
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    $\begingroup$ Hmm, and I thought old timey coffins just had funny angles to save on wood. Turns out the vamps just don't like 90 degrees. Next time I run into one I'll attack him with a box. $\endgroup$ Commented Apr 14, 2016 at 5:37
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    $\begingroup$ Amusing answer, but a bit of a stretch. I read that book a few years ago, and really enjoyed it. I don't remember single right-angles being a problem, only crossed lines so there are four right-angles together. A vampire could just look at the floor, or to the side, and see just the vertical parts of the door frame, not the right angles it makes at the top. $\endgroup$ Commented Apr 17, 2016 at 2:03
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    $\begingroup$ Wouldn't there be just as many right angles inside a house as outside, though? I just counted over 300 right angles in just the room I'm in right now. $\endgroup$
    – Timpanus
    Commented May 1, 2016 at 11:58
  • $\begingroup$ Sure, the point is that a single victim can be overcome even without sight. The vampire has to get inside a house to kill out of view, because he cannot expect to escape without seeing a thing if he draws attention. $\endgroup$ Commented May 2, 2016 at 8:07

It's a partially misunderstood half-truth

Short Version:

Various chemical compounds traditionally used in incense and other religious ceremonies are highly allergenic, even occasionally deadly to Vampires. At the dawn of Christianity, a practice developed to sprinkle the houses of the believers with these 'blessed' substances at least once every year. Those wanting to deal with the undead would have to purify their homes and remove all traces of such compounds, before these powers would deign to enter their homes. Hence, "inviting the devil into your home."

Full Story:

Speaking as a Transylvanian native, I believe I am quite well-positioned to answer your question.

The origins of the Great Prohibition (as it is known among the unliving) date back about seventeen hundred years, when a Sublime Alliance was formed under the great Undead Prince Atilla the Hun(gry), who then conquered much what of is now Europe and then led a cleansing invasion against Rome itself. It was a thing of beauty, decadent Roman army after Roman army collapsing under the unliving onslaught, with Vampire princes leading vast armies of Ghouls and Alghouls against the living.


Ahh. Sorry, got lost in reminiscing there for a moment. Yet the hour of our greatest triumph carried the seeds of destruction within. The Pope, who at the time was a puny little man going by the name of Leo, subsequently called by historians "Saint Leo the Great", received word from certain Northern Shamans that were appalled at the brazen Unliving intrusion into the world of the living, breaking with the subtler approach of countless centuries.

As it turns out, a certain molecular compound, trace amounts of which are contained in Myrrh, Frankincense and other rare compounds are likely to cause a severe autoimmune reaction in the modified globulin compounds present in vampiric blood, such that even skin contact or trace amounts on wood would cause the mother of all allergic reactions.

The Pope met Atilla on the road to Rome. Historical tradition has the pope begging for his city, but that was mere face-saving by the Hun. In fact, the Pope revealed that all homes in Rome had been doused in Frankincense and Myrrh, making the city literally lethal to approach.

As you can imagine, the practice quickly spread, with Atilla himself murdered only months later by the expedient method of having his last 'wife', an Ostrogoth princess called Ildico, perfumed in 'holy' water before being sent to his tent to be devoured.

More such indignity followed, and The Hun empire collapsed like a house of cards in the face of such potent bio-weaponry. The unliving reverted to the subtler approach of older times, and all was well with mankind. The 'blessing' of houses has since ceased (thankfully) for the largest part, but is still occasionally done in Orthodox communities: priest w holy water
Smug priest splashing deadly poison around

This simple practice made European homes inaccessible to the unliving for a millenium and a half. Of course, witches and warlocks that still had dealings with the unliving would have to take special steps to cleanse their homes of such trace fragrances, which later evolved in the popular lore as the concept of "inviting" the devil in your home. Silly, really.

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    $\begingroup$ I'll come looking for your answers whenever I'll need undead intel. $\endgroup$
    – Liquid
    Commented Aug 28, 2017 at 9:25

Determination to adhere to traditional rules of courtesy

The world's maybe most famous vampire in literature received his guest with the well-known words:

Enter freely and of your own will! Come freely. Go safely.
And leave something of the happiness you bring.

It is usually agreed that Stoker's intention behind this solemn wording was to make the Count appear unworldly and to give him a testimony of ancient times long past when these words were presumably a formal greeting that described a certain code of conduct both for and towards guests, which indeed exists -- in many variations -- in several ancient (and present) cultures.
Traditionally, anyone violating the laws of hospitality (Atreus and Thyestes come to mind) was considered damned1.

This code of conduct demands, among other things, that a guest may enter and leave your house freely and safely, unharmed. It also requires the house owner to give shelter to a wanderer, and it demands that one does not enter a home before giving the host a chance to speak out this invitation.

Since vampires are thought to be immortal, it is reasonable to assume that they are, on the average, rather ancient, and so are their memories and their conduct. It is thus reasonable to look at them much in the same way as one would look at a medieval paladin -- following such a code of conduct, and merciless submitting oneself to its rules regardless what the consequences may be.

Although highly doubtful, we know from the tales of old that a knight would rather perish than show cowardice, rather burn than tell a lie, and rather endure the worst punishment than break an oath, or fail to show justice or mercy.

In the same sense, one should assume that a vampire could most certainly, and easily enter a house without being invited. However, this would violate their code of conduct in a sheer unimaginable way, and they would rather perish than do that.

1Which is a somewhat ironical thing, seeing how the subject is on vampires.

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    $\begingroup$ And those few "new" vampires without such a code of conduct would leave fewer survivors, so the story would spread that you must invite a vampire in because those vampires who don't follow that pattern leave no one to tell the truth - that they can just walk in. $\endgroup$
    – Marsh
    Commented Apr 13, 2016 at 17:08

The vampire could merely choose not to enter without permission. Not even by compulsion, but by rational thought.

Perhaps the "curse" reveals the vampire's true nature if they were to enter their prey's home uninvited. The element of surprise is critical to a clean and efficient hunt. Should the alerted prey somehow escape, the vampire's presumably hidden identity could be revealed to the community. A vampire might wisely choose to wait until they're invited and can approach their prey without suspicion.

A code of honor could also drive this kind of behavior. Brute force is violent, messy, barbaric, and far too easy with the vampire's superior strength and resilience. A vampire worth their mettle earns their meal by subtler, more sophisticated means. It's easy to believe that vampires who default to assaulting their prey are considered improper, rude, immature, simple, backwards, etc.

Playing the game could enhance the flavor or actual effects of feeding. Pheromones used to subdue prey might only work if the prey is both calm and very close for some amount of time - impossible to achieve without being welcomed into proximity. Those same pheromones might also drive biological changes in the prey, e.g. releasing serotonin or other pleasure chemicals into the blood, or triggering metabolic processes that make the blood more palatable or digestible. The prey's emotional state might affect the taste or effect of their blood, so most vampires probably prefer to avoid heightened emotions (or even attempt to rouse very specific combinations of emotions for a specific flavor).

Being typically very old and worldly, perhaps they are just bored and looking for a challenge. Or maybe they're psychopathic and can only find the pleasure of cordial social interaction in the people they prey upon.

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    $\begingroup$ Stardust had a similar idea about the emotional state of the victim immediately before eating them. The fallen star/woman was most valuable to the witches if her heart was cut out just as she was happiest and at peace. It was still good if she died in terror, but it was so much better if she died calm and happy. $\endgroup$
    – Marsh
    Commented Apr 13, 2016 at 17:12
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    $\begingroup$ Commenting on the suggestion that the flavor might be enhanced by "playing the game" - I know many people who hunt, and they agree that an animal that is shot in such a manner as to cause immediate death, without startling it beforehand, has a better flavor than one than was pursued, or one that was wounded and took a minute to die. They attribute this to the stress of the death causing chemical changes in the meat. $\endgroup$
    – swilliams
    Commented Apr 14, 2016 at 17:41
  • $\begingroup$ @swilliams I've heard the opposite, actually - that an animal that was "well run" (ie ran in terror for several minutes) tastes better. $\endgroup$ Commented Jan 26, 2021 at 14:43

This answer by Philipp suggests a world where doors are operated electronically, and vampires don't trigger the sensors to open the doors the same way that humans do. The only way to get through the door is for someone else to open it for them (essentially "inviting" them in).

In his scenario, the vampires are actually invisible, and simply psychically project an image of themselves, which explains why they don't trigger the sensors (and why they don't appear in mirrors as well), but it could be something different as well.

  • Perhaps vampirism is caused by a virus of some sort, and doors are programmed to detect the virus and refuse to open to anyone carrying it, to prevent an epidemic.

  • Perhaps vampires give off (or reflect) some type of radiation invisible to the human eye, but which blinds the door sensors. (e.g. infra-red or ultraviolet rays).

  • Perhaps the door sensors do a retinal scan, or some other type of biometric scan, and the vampires are altered enough from humans that they don't register as human on the scan.


I hate to bring the Bible to a science-based question, but the answer to this one could be biblical; specifically, Exodus 12:23, where lamb's blood upon the door frame instructed Death to not enter.

Perhaps in a world where vampires are governed by scientific processes, biblical phenomena are also supported by (as of yet undiscovered) scientifically-measurable causes. If so, and if the force which animates the undead is be strongly associated to the force of death which struck down the first-born Egyptians, then the ancient science of that lamb's blood might bind vampires as well.

It would require a little tinkering with history to spread the affect across all doorways. Maybe an Egyptian vampire wandered through the front door of an abandoned Israeli hut soon after the exodus and got incinerated by the leftover lamb's blood. Ever since that day, all vampires have avoided family doorways out of superstition and fear, not realizing that the danger was in the blood, not the doorway.

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    $\begingroup$ Good point. I guess the original vampire stories are set in a world where the Bible is true. (Crucifixes and holy water working etc) $\endgroup$ Commented Apr 14, 2016 at 4:23

Vampires are extremely territorial. For a vampire to enter another vampire's territory uninvited is an act of naked aggression that will be met with immediate and unrestrained force. This generally benefits humans, as the secretive nature of vampires means they cannot be sure which homes are claimed by another, unknown vampire, who might ambush them while they are vulnerable in the act of feeding.

The invitation serves two purposes. Firstly, it serves to soothe the vampire's instincts that warn against leaving its own territory. Secondly, it indicates to vampire that the home is not part of another vampire's territory.

Through subtle hormonal alteration applied directly into the bloodstream, vampires affect the dispositions of the humans upon which they feed. A drawn human will become reclusive, suspicious, and share the vampire's extreme territoriality. The invitation to enter, given without duress, is a clear signal that the human does not live in a dwelling claimed by another vampire.


Self-hatred: Perhaps the vampires actually despise their own actions and what they have become. In an act of self-justification, they feel that if they are invited in that they have permission to do what they need to do. Sort of like a member of an organization that hazes initiates and justifies it by saying "That's what I went through when I was new". So the vampire thinks about their decision to allow entrance to the vampire that turned them, and justifies it: "It's not my fault. They bring this on themselves, as I brought it upon myself."

Lack of power: Perhaps vampires really don't have any great strength, can't turn into mists or bats and fly, etc, etc. They're actually rather sickly except that they have fairly strong psychic powers. Not so strong as to be able to overpower someone who stands up to them with a clear mind, but strong enough to overcome the weak or clouded mind. Considering their strangeness, most people get a "bad feeling" about them, but those who are weak or inattentive enough to invite them in are weak enough to be overcome psychically.

A test: Perhaps vampires can control some minds but not others. It's a genetic thing, like having allergies. The request may be anything unusual, not just entering a house, and it's a test to see if they can control the intended victim or not. So it might be, "Give me $100", or "Let me have your horse", if you're out in the middle of nowhere, but inside a home is a convenient place to commit vampiric crimes, so the house thing is traditional and useful.

Drugs: Perhaps vampires have no psychic abilities at all, but rather rely on a psychotropic drug, and the request is a test of whether they were able to administer a sufficient dose. Administering it is an art (in the air, in food, etc), and you can't administer too much or the victim passes out in public, or perhaps dies and is toxic, so you need just enough to assure compliance, but not so much as to cause trouble or spoil the meal.

Last, are you sure it's entering another's house uninvited. Wasn't the famous movie line about the victim entering the vampire's house of their own free will?


I like the psychology angle. Consider that perhaps they can, but they just don't. Vampires are traditionally vile creatures, and there's a certain poetry to the limitation. It mirrors a condition in the human mind of inviting one's own troubles upon oneself.

It's a trope in fiction, really. At the end of the story, when the hero has lost and the villain has won (or at least it so appears), the villain gloats that the hero was in part responsible for the outcome, often using lines such as "you invited all this on yourself" or "you have nobody to blame but yourself."

As the traditionally vile and poetic creatures that vampires are, maybe they appreciate the irony and use this strategy of only entering when invited to ensure that their own desire for drama (as well as food) is fulfilled.



Could the 'love' in a home be unbearable for the vampire, unless an act of kindness is extended from the people in the home, i.e. being invited into the home.

Love does include chemical processes in the brain. And some left-behind empathic wiring in the brain of the vampire could go into a very painful overdrive, that could be cancelled by the invitation.

I know this is supposed to be scientific; I hope this is sufficiently scientific.

  • $\begingroup$ What is the mechanism by which the vampire would be sensitive in this way? How would an invitation shut it down? Do all homes have this kind of "love"? What if no one is home? Why would this be specific to homes, anyway? $\endgroup$
    – mattdm
    Commented Apr 12, 2016 at 12:32
  • $\begingroup$ When turned into a wampire the brain is fried. The only warm thing left is a memory of your childhood home and that safety that was there. At the doorstep the wampire knows it is about to destroy that safety for others and the overdrive of the earliest memories kicks in. Being invited into the home breaks the overdrive because then everything is ok and safe. (But then what about wampires that was orphans or had abusive parents...). $\endgroup$ Commented Apr 12, 2016 at 19:24

This is your best bet :Radiation emitted by the human body

Yes, all objects, including human bodies, emit electromagnetic radiation. The wavelength of radiation emitted depends on the temperature of the objects. Such radiation is sometimes called thermal radiation. Most of the radiation emitted by human body is in the infrared region, mainly at the wavelength of 12 micron.

  • A vampire can't enter a house when he senses hostile emitted radiation (not welcomed).
  • A vampire can enter a house when he senses friendly emitted radiation (welcomed).
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    $\begingroup$ Although this wouldn't cover someone who wasn't particularly hostile but hadn't explicitly invited them in (e.g. someone who didn't know they were a vampire) $\endgroup$
    – komodosp
    Commented Apr 12, 2016 at 9:59
  • $\begingroup$ Doesn't explain people walking down the street either. Or, for that matter, what if we had a setup like The Truman Show? That is, a huge building that contains inside it smaller buildings that resemble a small mid-american town. What constitutes "inside"? $\endgroup$ Commented Apr 12, 2016 at 16:07

What about the Extended Phenotype shown by certain parasites?

Parasite Manipulation is where a parasite invades a host and then forces it into behavior outside its norm.

Examples of this are:

  • the suicidal drowning of crickets infected by hairworm, a behaviour that is essential to the parasite's reproductive cycle.

  • female mosquitoes carrying malaria parasites. The mosquitoes are significantly more attracted to human breath and odors than uninfected mosquitoes.

  • ants infected by a fungii called cordyceps. These ants will move down onto the jungle floor out in the open (a complete anti-survival trait for them) before becoming immobilized by the cordyceps which then bursts its spoors over a wide area to infect more ants.

In short, your vampires might be hosts to some killer virus that alters their metabolisms to feed off of blood. As a bonus, it heightens their senses, increases their reflexes and strength and their ability to heal.

In payment, it 'abhors' certain kinds of smells, perhaps the pheromones emitted by uninfected humans when they feel safe cause the virus to avoid the area and instead prefer areas where humans are emitting sexual pheromones (such as near a bar or maybe where prostitutes hang out). This would be way beyond deep at a near instinctual level and would be extremely hard for an infected vampire to fight or ignore.

  • $\begingroup$ This is an interesting idea, but it misses an explanation for why inviting a vampire in would suddenly allow them to come in. $\endgroup$ Commented Apr 14, 2016 at 1:17

Why do vampires have to be invited in? –SciFi.SE

It depends on what vampire literature your reading.

(below is my deleted answer to the linked question)

The fifth tradition: The Hospitality

Honor one another’s domain.

When thou comest to a foreign city, thou shall present thyself to the one who ruleth there.

Without the word of acceptance, thou art nothing.


Greece is one of the oldest sources for the contemporary vampire legend. It produced the first modern writer on vampires, Leone Allacci, in 1645.

Xenia is the Greek relationship between two people from different regions. [Hospitality] allowed for the members of the relationship to safely travel into the other member’s territory and receive a place to stay and something to eat. -The Value of Hospitality

It's the same reason Highlanders don't fight in church. It's not that they can't, they just don't. Conduct is very important when you're immortal.

So, entering one's house without permission is a faux pas that they won't commit, but they'll murder the same person? Vampires have a very strange definition of "polite". – phantom42

The host is honor-bound to sustain (feed) you once you're invited in. Proper guests do not disrespect their host by declining dinner, thieve, nor break and enter.

My point is that once you invite them in (they can't break and enter anymore), it's no longer murder and they can no longer thieve the sustenance they need from you, as it has been freely and automatically offered up due to the standard protocol of hospitality.

"Welcome to my humble abode," is the equivalent of signing the dotted line on a contract with the devil.


Vulnerability: The vampire needs the invitation for its own protection.

(Admittedly, this idea sprang to mind after seeing some of the options listed by Liesmith's answer.)

It could be that humans leave behind some sort of evidence of their residence; either a scent, or something similar that supernatural beings can detect with a sense we don't. However, just as the substance of oxytocin is claimed to affect trust and hormones are a real physical trait that impact people's emotions, this is sort of the same concept (possibly going in a reverse direction) -- the real physical acts of letting down one's guard and entrusting a person cause a physical impact on the world. Humans are not quite so affected by the resulting aura, but a supernatural creature does benefit.

Sleep tends to cause one of the ingredients in the air (which might combine with some other ingredients to cause the barrier), so places like homes and hotel rooms are saturated with a protective element that you don't find in a sports arena or a grocery store.

This might be combined with Liesmith's concept of Legal impact. The idea of the "legal" threat could come from peers like he mentioned, or some other supernatural power. Perhaps vampires know that their actions, considered horrendous by the victim species known as humans, are actually condoned by divine ruling. The vampires know that their actions will be traceable, because biting a human will leave a residue which is easily traceable using supernatural methods (or even a method that a natural being can use, if they have the supernatural equipment and/or know-how), so the vamp cannot effectively deny being in the presence of the human.

Alternatively, instead of a legal threat from another species, the chemicals in the air could reduce the vampire's strength or increase its mortality, so that even a child could kill a vampire. (This might be caused by a side effect; if the vampire violates the supernatural law, not only is he weaker than a child, but the resulting air chemical is likely to cause intense hatred within the human, causing an otherwise-normally-peaceful human to want to kill any supernatural creature, even if the human doesn't rationally know that the creature is supernatural. Such a setup might be useful for one vampire who exploits the rules to get another vampire in trouble. By violating the rules, he causes another nearby vampire to be detected as the vulnerable recipient of lethal hate.)


My two cents: Vampires are politicians/salesmen. Yes, in the concept of world building one needs to accommodate the physical nonsense of vampirism such as how much blood is needed for a vampire to live, where they sleep, what kind of capes they wear and all that superficial bullshit.

But looking at the historic presentation of vampirism over the ages, all of the behavior comes off as classic “wolf in sheep’s clothing” politician/salesman behavior. And if you think about the value of legends and myths in a society, it’s fairly easy to see someone describing a politician/salesman as someone who is a “monster” that “promises you everything but sucks you dry” has a “insatiable appetite.”

Yes, there are some politicians/salesmen that use bullying tactics and brute force to get what they want. But are they more or less common than politicians/salesmen that convince their next “victim” (aka: client, market, constituent, sucker, etc…) that they want what they are selling? And in general in life, isn’t it easier to get someone to do something for you as long as you consider it’s being done in their own best interest?

If a vampires hungry, just go and attack some schlub at a tavern, drain him dry and leave his corpse for the rats. But you want to recruit a new vampire for your army of underlings? Befriend them… Seduce them… Promise them eternal life… Let them live forever! As not only a monster, but a slave to a larger monster… They’ll get what they wished for and much, much more!


This is the result of a blood pact with humans.

A long time ago, so much that noone but a few of us remember it, when vampires (and humans!) were young, there was a fierce war amongst the two races. It ended with the Peace of the resting places, and one of its most important provisions was that vampires would no longer enter into a human home (unless invited to do so).

(It also forbid humans from killing a vampire during the day, but they were not so constrained by the Pact and breached it)

The tales of humankind tell that they had powerful wizards amongst their side, that led them to the victory, and ―after vampires surrended― imposed such spell upon them.

The informed reader will inmediatly recognise this as pure fantasy. There's no way the human forces (already quite weakened after the many deaths just for fun by some irresponsible vampires, which ultimately led to the cities joining against us in such war) could have defeated the vampire army. And even if such thing could have occurred, it would have been a very stupid move, should they have held such power. The complete annihilation of vampires ―which was their original goal― would have been a more sensible move. Or had it resort to such “magic” prohibition, not killing any human or banning entrance to any city would have been much wiser restrictions.

Truth to be told, vampires could have easily wiped out all the humans. In fact, the problem was that they would have wiped out all the humans. Leading to starvation of (most) vampires on the upcoming winter. Presenting the result as a defeat, ending with this "peace treaty" was a genius movement by our leader:

  • it made people felt secure and diminished their motivation to fight back
  • rebellious vampires would not be able to indiscriminately kill food, as it would be safely stored at their homes
  • at the same time that it only provides a weak protection from us to human beings

not to mention some other perks that human agreed to provide us as tribute :)


The assumptions here are that

  • A vampire can't break a blood pact

  • Due to their vampiric nature, blood pacts are transitive. You are affected by those affecting the vampire that converted you. As this event happened in the early beginning, it affects pretty much to all the vampire community (but leaves room for having a few vampires immune to it).

  • $\begingroup$ While this gives a rational for "why" it doesn't explain the "how". A just how would a curse achieve the goal of stopping vampires entering uninvited. (Note that this is looking for science based vampires, not magical fantasy ones). $\endgroup$
    – Tim B
    Commented Apr 15, 2016 at 11:43
  • $\begingroup$ @TimB I have updated it expanding a bit the assumptions $\endgroup$
    – Ángel
    Commented Apr 15, 2016 at 13:53

It could be a spiritual obstacle, since vampires are creatures of pure 'evil' and a home is a place of spiritual strength and warmth. Once the owner gives permission for 'evil' to enter the home, the spiritual lock is broken. Similarly the vampire reaction to the crucifix is an extreme reaction of a pure 'evil' being to an object holding strong powers of 'good'.

  • $\begingroup$ Hmm, so maybe only a home with a hearth (fireplace) will keep them out. That could be interesting to figure out how/why vampires start getting into more modern homes! $\endgroup$
    – JDługosz
    Commented Apr 12, 2016 at 19:21

Several authors have said that their vampires have a problem because of the innate power true homes acquire over time. This leads to the implication that the less time you spend in the home, the less power in its domain it has. In fact, Jim Butcher shows a demon trying to get into his home and notes that the power of his home would hold off the demon for a little while but not forever.


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