The synopsis on the back of your book or in the middle of your query letter is the gateway to getting readers. You must grab their attention and then hope your story doesn’t let go. Advertisers have much the same dilemma. What grabs attention? What causes people to skim over certain parts? Here are the determinants of attention.
There are external determinants and internal determinants. Let’s start with the external.
The stronger the stimuli, the more attention it attracts. Louder, brighter.
When converted to writing, I think that would translate to more dramatic. The more intense your dramatic events, the more attention it will attract.
The bigger the stimuli, the better. Bigger things, like advertisement boards, attracts attention better than tiny flyers pasted on walls.
What this could mean for us, I can only guess. Perhaps the bigger your climax or inciting incident, the more attention it will demand.
Closer stimuli will be more prone to catching out attention.
The fact of the matter is, no matter how good the synopsis is, if no one picks it up, no one will read it. Your synopsis should begin with a bang, be interesting enough that the reader will not just throw it away. Even the title can affect how far away your synopsis will be.
Movement in the field of vision, as well as unexpected and odd stimuli catches our attention very easily.
This one is easy. New and exciting things will grab attention like nothing else. Original ideas etc.
Repetition of stimuli can establish an idea quite well, but there is always the danger of habituation (getting used to it and thus not noticing it any more).
Repeating important points (maybe something like symbolism) can often drive home an idea or the theme, but you must have something original to counter balance the habituation. A tricky situation, but perhaps worth it if you rely heavily on symbolism in your story.
In general, complex stimuli are more interesting, but it is entirely dependent on the person on the receiving end. Every person finds different amounts of complexity attention-grabbing.
Don’t make your plot seem too complex or too simple, except if you know your audience will respond to a specific one.
Every person is more likely to notice something that is personally relevant to him or her. Let’s say you are learning a new language. Suddenly, you’ll be seeing more of that language in random adverts and magazines. Not because there was a sudden influx, but because you now see the information as relevant and therefore your mind fixes your attention on it.
It is important to know your audience and what they find interesting. Use those points to attract attention.
To find readers (or customers) you have to grab them from the get-go. The same can perhaps be said of the hook, but a synopsis, especially if a publisher or agent is reading it, is a very important part of your toolkit to getting attention. Use it.