Both of those most likely have some underlying genetic causes. Both of the animals can be venomous, so our ancestors may have been genetically "programmed" to avoid them. That would explain why people in countries that have only non-poisonous spiders or snakes still often are afraid of them. Similarly many people are afraid of heights even in situations where there is absolutely no chance of falling down.

However, there certainly is a learnt element involved: children of people who are afraid of something usually learn from their parents to be afraid similar things. And since women generally express their feelings more visibly, daughters quickly learn to be afraid of spiders because their mothers go EEEEEKK!! every time they see one. Men usually try to keep their cool and sons hanging around easily learn those things are not scary.

Arachnophobia is learned behaviour. Put a very young child next to a spider and they won’t bat an eyelid.

It is not that simple. Very young children don’t do pretty much anything. They do not feel sexual urges towards opposite sex, something that is certainly genetic as well. Like I said earlier, arachnophobia is likely to be both genetic and learned, just as much of the human behavior. For example, speech is also a trait that includes a strong genetic element in addition to the definite need to learn it.

"Irrational" fear of (non-venomous) arachnids, snakes, insects and all other things out there is such a worldwide phenomenon that it almost certainly has also a genetic basis. It is only logical that people naturally avoid or shun animals that could be poisonous or parasitic or disease carriers, even though these animals are physically and mentally so inferior to humans that there is no obvious need to fear them. Apparently it has been evolution-wise better to be somewhat afraid of everything that resembles venomous animals, even if it meant that you fear spiders and snakes even in a country where only completely harmless species live.

Humans, like virtually all higher vertebrates, adjust their underlying genetics by learning: thus people can learn to be not afraid of spiders if they handle them a notice they are not dangerous. Or they can become very afraid of something after a single adverse experience (like being stung by a wasp as a child). The "natural" genetically programmed stand towards spiders and such is probably somewhere between those extremes: a natural cautiousness towards things or animals that are not proven to be safe, affected by the general mindset of the person in question (from curious to timid).

biohazard: are you sure, that * people in e.g. Africa or elsewhere are afraid of (non-venomous) arachnids etc.?

* native/primitive? because of lack of correct English word, I mean people who live in wild nature like Indians in Amazonia or the people in Africa, which I don’t know the name of.

I don’t know if they are or not. And like I wrote, people learn. They learn to be afraid and they learn to not be. If they get into contact with non-venomous spiders all the time, they probably couldn’t care less. Many European or American people come across with spiders now and then, but generally have such a bad knowledge of biology that they don’t even know if it is dangerous or not. And even if they did, they do not really have any reason to change their behavior. In the West it is mostly women who are (openly) afraid of spiders and such, because it is generally regarded almost as something women should do. This still does not mean there wouldn’t be a genetic basis behind it.

If you go deep enough, all human behavior is genetic…

If you think about it, many people are afraid of surprisingly many animals. Many are afraid of rodents and reptiles in addition to insects, which are potentially venomous, parasitic or disease carriers. They are also afraid of big animals (unless they are domesticated and known to be safe) that can kill them or injure them. So people are pretty much afraid of everything that could have been dangerous to their early ancestors. Pretty much only small/medium sized mammals around the size of a hare or a fox, and most birds, seem to be almost universally no-so-scary, as well as some other animals that can easily be distinguished from all dangerous animals (say, butterflies).

A modern human, as well as the early ancestor, could usually overcome their fears if they had to. A lone caveman would probably be very afraid of an angry mammoth or a pack of wolves, but together as a group they’d hunt them for food nonetheless.

Modern people just have no real reason to overcome such fears. Easier to go eeek! when there is a spider somewhere…

p.s. there is a distinction between fear and phobia. Fear is probably the normal version of the reaction that is supposed to take place in potentially dangerous situations. Phobia is irrational fear even when there is absolutely no way for anything bad to happen. Many people who have one phobia have other phobias or irrational fears as well. It seems quite unlikely that they simply learned all their fears from other people (though a hysterical mother could do a good job there), but instead they are genetically prone to overreact on any possible danger, real or imaginary, there is.

kids are all different. its adults that are forced into the finite number of pigeon holes.