Choosing a knife for self defense

This question comes up once in a while; if you can legally and the situation allows, a firearm should be your first line of self defense weapon. However, sometimes laws and situation dictate a blade as a primary. And, even if you're packin', a knife can make a great secondary or tertiary weapon. A knife is an essential tool for everyday and TEOTWAWKI survival scenarios as well, so having a knife that can pull double duty between chores and defense is a good thing, too.



When selecting a defensive knife, here's some things to consider:

Fixed, folding and opening mechanisms

An Emerson Super CQC-7; the "wave" hook on the
blade's spine catches against your pocket, pulling the
blade open. This knife could also be opened with
a simple flick of the wrist when needed.

• Fixed blade knives are much faster and require less fine motor skills than folding knives. Fixed blades are stronger than folding blades. If you can, go with a fixed blade.
• If you can't carry a fixed blade (many jurisdictions frown on 'em), then you need a folder that you can deploy quickly and with minimal fine motor skills. The Emerson Wave is an excellent workaround and is recommended for a folder that is going to be primarily for defense. 
• Beyond the Wave, I also find knives with flippers to be easy to open with minimal fine motor skills - Kershaw and Zero Tolerance folders often have these flippers. Many liner locks can be opened with inertia - a flick of a wrist - which is also easy to do with minimal fumbling.
• As an odd-ball, balisong (butterfly) knives with a spring latch are also very fast and simple to open if you know what you're doing. The balisong is also a very strong knife design. They're usually of questionable legality, though.
• Beyond those, it's a toss up between thumbstuds, spyder holes and thumb discs. The size of the knife and the ergonomics are usually more important. A small-ish knife with bad ergonomics will be harder to snap open under pressure. This is critical - if you can't get the knife open, it will at best be a blunt force trauma weapon.
• Continuing the above, if the knife is a folder, make sure that it could be reasonably used as a striking weapon, especially if it has one of the "slower" opening mechanisms. Some heft to it, some solid handle material, maybe some point to it - basically, something that could do some damage, even closed.

Build

• Make sure the knife has a sturdy locking mechanism and is of good quality. Buy knives of known quality, from reputable makers, not pot-metal Chinese stuff from the flea market.
• A quality blade steel is important, especially if the knife will be doing EDC chores as well. Blade steel is less critical if the knife will be carried only for defense purposes, and thus rarely (if ever) used.
• The blade should be pointy and capable of stabbing, and it should also be sharp enough to be capable of slashing. Overly rounded, blunt or squared tips are out.
• The blade should not be overly thin/fragile. You do not want it to snap in a time of need, though an thick blade it not as important on an SD knife as say a bush crafting knife that you will be batoning through logs, prying with, etc. A SD knife needs to stab and cut through clothing and flesh, so the blade needs to be up to that task. 
• Double edges are nice on a fixed blade - they give you cutting options without the need to shift grip - but daggers and double edged blades are usually a no-no with the law.
• You don't need a gigantic knife for self defense; even a small knife can be effective in dissuading an attacker. Avoid compromising on size more than you need to for legality, concealment and comfort purposes, though.

Ergonomics/Carry

With the addition of an aftermarket sheath,
the Izula II makes a good defensive knife.

• The handle/grip should be ergonomic enough that it can be comfortably and securely held through serious use.
• Added retention is good - finger holes like on the La Griffe, karambits and a variety of other knives are a very good addition. The T-handle of push daggers is also good, though they often fall on the wrong side of the law.
• You must be able to carry the knife in a secure, quickly and easily accessible way, and in a way that provides a constant position to allow for a consistent draw. For folders, that usually means a good pocket clip. For fixed blades, that means a good sheath. Unfortunately, most from-the-factory sheaths are absolute crap, so plan to spend some extra money on kydex when you pick up a SD fixed blade.
• Practice drawing the knife regularly, until it becomes second nature. The draw should be smooth, fast and require minimal fine motor skills and and wrist articulation/bending.

You'll see that there are a great many knives that fit into my criteria, which is because a great many knives will work well for self defense. If it stabs, cuts, is of good quality and can be deployed quickly, it will probably work. Some knives are certainly better suited to defense/combat than others, and personal tastes, lifestyle and local laws also play a role in selecting a self defense blade.

My current rotation of self defense knives includes a Spyderco/Shivworks P'kal, a Ka-Bar Small TDI, and an  ESEE Izula II. I've been eyeing a few new knives to add some variety, so I may be adding to the stable shortly.