I guess it is a good marketing ploy, but for me it doesn’t always work that way.
But these people still annoy me when they do it. I discovered it done to me again today.

Have you ever bought a book that, in big letters near the title, says “Volume No.1″ ?
That usually means that other titles will be coming out in due course. And the sneaky publisher next brings out Volume No.4 because the previous two volumes are still in the pipeline. As soon as you have bought Volume No.4, you are trapped! Sitting up there on the bookshelf together, the set is incomplete.
Now psychology works against you.

We are all wired up in such a way that there is an innate drive towards completion - to fill in the missing bits to make something into a whole, or in psychology, a gestalten.
The way our perception is configured, we all have a strong tendency towards seeking closure, continuation, proximity, similarity and common fate among elements in our experience.
An obvious demonstration of this tendency is in the difficulty of detecting a simple little spelling mistake in a paragraph of one’s own writing. It is always better to get someone else to check instead, or to use a spellchecker. And there is another interesting example where you find you can still read and understand a paragraph of text that is written with all the vowels removed. On the basis of this ability, we can abbreviate words when writing hurriedly and still understand the message at a later date.
Any given nanosecond, we take in great dollops of data and make sense of it by being able to group it into an identifiable context. Or we take in just enough of the data to make sense without uploading the whole of it. Without this ability, we would likely be dead from mental exhaustion before we had even learnt to walk!

So, where is all this heading?
Back to the bookshelf, and to Books No.1 and No.4 with your natural question about to follow: “Where are No.2 and No.3?”
Does that make you want to get those missing books as well? The fact that they are deemed missing is a give away. How happy are you with something that is incomplete? Well, that is all a matter of personality and vast individual differences, but the marketeers have certainly got psychology on their side when it comes to that.

I had a little conversation about these matters with my son some years ago. As a young child he was falling over himself to amass the entire collection of those awful Pokemon toys. We were visiting the USA at the time, in one of those huge toy barns full of these things, and I wanted him to count the number of variations of one item. Just then we became witnesses to a spectacular display of temper from a young member of another family in there. Dad had apparently limited the choice to only one item, but young kidlet knew that they came in a set of five and he wanted all five because just one was completely unbearable to him. My son and I stood back and watched. There was considerable drama until Dad got hold of himself, and then of his youngster, and marched the kidlet right out of the store. After that we had some interesting discussion on what constituted a set, who decided such things, and just how much they might benefit from doing so. My son’s collection kind of waned after that, and he became far more discerning with his little bit of pocket money.

Now I am definitely not going to invite you to look in the drawers of my sewing desk at all my acrylic quilting templates. If you do, you will have to understand that I actually wanted every one of those! Yes, even the ones I hardly ever use… or have never used at all (er, yet, that is).
After all, my collection would hardly be complete without them!

Changing the subject…
We were out for lunch today and I noticed everyone at adjacent tables - yes, absolutely everyone! - on being delivered their meals, picked up the salt and sprinkled it liberally all over their food. Good grief, they didn’t even taste it first!
How funny it would have been to have given them a dish already so highly seasoned that any more salt would have made it inedible.
No wonder restaurateurs place a big jug of water on each table. But what about all those poor kidneys? And how much escalating hypertension was dining out as well?

Just a thought.