When can you use deadly force? The short answer is that your attacker must possess the ability and opportunity to do you grave harm—but you must also have to have done everything in your ability to avoid a confrontation.

In the new Gun Digest book Armed: The Essential Guide to Concealed Carry, author Bruce Eimer, Ph.D., provides more insight to this point:

When would the use of deadly force by a private citizen against another human be considered judicious? How can a private citizen be authorized to kill another human under his or her own summary judgment? The very simple answer is that deadly force is recognized as a last resort for use only when you need to save your life.

We are referring to the “doctrine of competing harms” and the “doctrine of necessity.” Put very simply, you are allowed to break the law (in this instance, kill), in the rare circumstances where following the law (not killing) would cause more injury to you or other innocent humans.

In reality, the answer is not so simple. Any time you even draw your gun in a social confrontation, you are walking on thin ice. If you are going to keep or carry a gun for self-defense, in addition to being well trained in marksmanship and tactics, you should be well-educated about the circumstances under which the use of deadly force is warranted legally and morally, so that you can be judicious.

The Ability to Do Great Harm

Here we are talking about the concepts of power and disparity of force. Clearly, a person with a gun or a knife and the ability to use it has the power to kill or cripple you. However, you can’t shoot that person unless he has the immediate opportunity to use that ability on you and he acts in such a manner that leads you to reasonably conclude you are in immediate jeopardy. But, what about if the threat does not have a gun, knife, or bludgeon? There are several other factors that would fulfill the criterion for ability.

1. Force of numbers. Two or more threatening persons, even without identifiably deadly weapons, against you alone, would constitute a disparity of force. If they attack you and act in such a manner as to lead you to believe that, unless you do something, they are going to kill or cripple you, then you are on solid legal ground. Against a group of attackers, each member of the group shares the same responsibility for the fear the group creates in the intended victim, and also shares the danger from the intended victim’s lawful response.

2. Able-bodied versus the disabled. If you are old and frail or physically challenged and you are viciously attacked by a younger, more able-bodied man (and the criteria of opportunity and jeopardy are in play), you are on solid legal ground.

3. Greater physical size and strength. If you are attacked by King Kong Bundy, you are on solid legal ground in using a force multiplier (a weapon) to avoid being killed or crippled.

4. Training or reputation. Is the attacker a person known to you to be highly trained in the martial arts? For this criterion to be considered a valid, affirmative defense for the defensive use of deadly force, you must have known about it before you resorted to using deadly force. It is not valid if you didn’t know it at the time, but learned that it was so after the fact. You will be judged based solely on what you knew at the time.

5. Male versus female. Our society assumes that females are more vulnerable and that there is a cultural predisposition for males to be more inclined to violent physical aggression than females. So, if you are female, you are being attacked by a lone male, and the other criteria of opportunity and jeopardy are in play, you are on solid legal ground in terms of using deadly force if you have no other viable choice to avoid being killed or crippled. This would also include self-defense against rape.

To read more about the issues surrounding concealed carry and use of deadly force, check out Eimer's new book, Armed: The Essential Guide to Concealed Carry.

Bruce N. Eimer, Ph.D. is a clinical psychologist, NRA Certified Law Enforcement Firearms Instructor, Certified Utah Concealed Firearms Instructor, and a professional writer. His daily work includes practicing clinical psychology, conducting forensic evaluations, writing, and training people in the defensive use of firearms through his company, Personal Defense Solutions, LLC. Doctor. Eimer has testified as an expert witness in civil, criminal, gun rights, and self-defense cases. He is the owner of the popular online forum, www.DefensiveHandguns.com and writes the monthly “Armed Senior Citizen” column for Concealed Carry Magazine. Doctor Eimer is the co-author of the Essential Guide To Handguns (Looseleaf Law Publications, 2005) and has contributed articles to various gun magazines such as Combat Handguns and www.GunsAndPatriots.com.