Our Inner Conflicts is one of the best psychology books I've ever read (possibly the best.) I am going to summarize it now (and, of course, recommend it), and will have more to say about it and her other books later on.
Karen Horney was a Freudian psychoanalyst: she's actually classified as a neo-Freudian, because her ideas departed significantly from Freud's [for example, he felt the drives were more instinctual and universal (such as the libido), whereas she felt they formed in the person in response to the specifics of the early environment (i.e. the neurotic trends I am about to talk about.)] Most of her significant books were published in the 1940s. Her work is very significant to the enneagram of personality because the Hornevian triads (the compliant, "moving toward people" types, the aggressive, "moving against people" types, and the withdrawn, "moving away from people" types) are based on her classification of these three strategies (although in some cases, the specifics of who falls into these types may be different -- for instance, while reading Our Inner Conflicts, I got the feeling that Horney would classify the 1s, or at least some 1s, as moving against people. (So, incidentally, does the writer of this article.) In case you're not convinced how relevant she is to the enneagram, take a look at a brief list of the neurotic trends:
(This list is taken from a previous Karen Horney book, Self-Analysis.)
1. The neurotic need for affection and approval
2. The neurotic need for a "partner" who will take over one's life
3. The neurotic need to restrict one's life within narrow borders
4. The neurotic need for power
4a. The neurotic need to control self and others through reason and foresight
4b. The neurotic need to believe in the omnipotence of will
5. The neurotic need to exploit others and by hook or crook get the better of them
6. The neurotic need for social recognition or prestige
7. The neurotic need for personal admiration
8. The neurotic ambition for personal achievement
9. The neurotic need for self-sufficiency and independence
10. The neurotic need for perfection and unassailability
Horney theorizes that many people develop a number of these neurotic trends. Some, such as the first three, may fit really well together; however, what if someone has several of these that don't fit together all that well? Then the person will not be able to pursue one thing whole-heartedly, and may try and try yet flounder repeatedly in love and work. (Take as an example a person who has a strong need to depend on a partner AND a strong need for complete freedom.) Horney sees the personality as being composed of these trends as well as elaborate structures created to hold competing trends together in such a way that they make some kind of sense. She says these strategies are taken up by the personality to keep the person from flying to pieces.
The four main strategies are: moving towards/against people, moving away from people, identifying with an idealized image, and externalization. 1. If my strategy is moving towards people, I will express, possibly over-express, anything pro-social, and I will bend over backwards not to do, say, or even feel anything that could be construed as aggressive. My aggressive impulses will have to come out in veiled ways. Similarly, if my strategy is moving against people, I will be horrified at expressing any weakness, crying, etc, even though I may also have a need for affection as well as other hidden trends. 2. Moving away from people. This strategy is different, in that neither the aggressive side nor the compliant side needs to be repressed; instead, the person withdraws from others so that he can maintain his integrity. 3. Identification with an idealized image. The person creates an image, then tries to fit reality to the image, then likes or dislikes himself (deep down) depending upon how well he is able to truly fit the image. 4. Externalization. Other people and forces are making me do these things and therefore, they do not conflict (since I am not responsible for them.) Related to projection.
Now keep in mind that I am not doing the material complete justice. Her writing is elegant, easy to read, and very insightful throughout. (Amazon lets you read a random page; try it.) Additionally, I am summarizing the book from memory (I read it a month ago).
So in the next section of the book, Horney describes some of the effects of having warring neurotic trends. Some of the effects are fears, hopelessness, and the impoverishment of personality. Impoverishment of personality involves inertia, indecisiveness, and fatique.
The vast majority of the book is devoted to descriptions of the personality structures and effects of the structures and trends, and I was left wanting more about how the trends came about and what to do about them. (Enneagram literature's strength is often the descriptions, as well.) As for how the trends arise, I am reading her other books in order to find out. Horney also feels that what is most important is not how they arose but what benefits you are getting from them now. (I believe she developed this from her clinical experience, as she saw people try to avoid facing their current problems by dwelling on the past.) As for how to improve, I think for Horney the answer would have been psychoanalysis (or, in a pinch, self-analysis.) But she does describe the goal, which is (as we've heard many times before) to be flexible (i.e., can behave appropriately to the situation, can choose from many ways of behaving rather than being stuck in one way for all situations however inappropriate), but also, to be whole-hearted. In other words, your neurotic trends (through analysis, or possibly because they weren't that strong or warring to begin with) are not strongly at odds with one another.
If you are very interested in the enneagram, I think you should definitely read this book. I have warring trends so I found it really fascinating for that reason, and I think my enneagram studies have helped me understand the book. I suspect that some people will find this painful to read, while others would be resistant to the ideas or even not able to understand them because they are too threatening. As I said, I will be writing more about her ideas in the future.