In my first semester as a graduate teaching assistant, I was assigned to a Modern World History course. Given that my undergraduate degree was in Latin American Studies, the first solo lecture I was asked to give was on Latin America; an easy enough theme to tackle -- or so I originally thought. That was until my professor more fully explained his expectations: "Just discuss the history of the region, you know, from the age of conquest until the present." In other words, 500 years of history . . . in 50 minutes.
Now, green as I was, I didn't know enough or feel sufficiently confident to question the feasibility of this task. Certainly I was overwhelmed, but I simply chalked this up to inexperience. As I began to develop my notes, though, my sense of false confidence grew; after all, I knew everything there was to know about Latin American history, didn't I? At the very least, I knew more than the students (and apparently the professor as well). I became engrossed in my preparation, taking down notes on everything from trade monopolies of the colonial era to emerging dictatorships in the twentieth century. By the time I was through, I probably had ten pages of typewritten notes, replete with demographic data and timelines spanning centuries. If this wasn't prepared, what was?
I approached the lectern that fateful morning prepared to dazzle and enlighten. I'd like to say that the class walked away with at least a passing knowledge of the overarching historical trajectory of Latin America; however, most staggered out of class that day almost as dazed and confused as I. Almost, but not quite. I blissfully remember little of that lecture -- sporadic, incomplete notations on the board, occasional memory lapses despite my reams of material, fitful glances at the clock as time ran short and I remained mired in the eighteenth century. Where and how did it all go so wrong?
With time for reflection, and some sage words of advice from my professor, I realized that I fell victim to overambition, feeling I both could and should share everything I knew about the subject. What I didn't realize at the time was that the students wouldn't retain most, if not all, of what I was trying to impart. I learned that the most effective method of lecturing is to limit yourself to three main points, the three most important ideas or themes that you want your students to remember. For example, I might have chosen to focus on the independence era, its aftermath, and the increasing influence of Europe and North America in late nineteenth and early twentieth century development. I would have limited the time range of my discussion, but greatly increased the chance that the class would have actually learned something.
You may be thinking, "That sounds fine for topics in world history, but how and why should I apply this rule to my lecture on the nitrogen cycle, or the rate of diminishing returns?" The how is simply part of your discipline in preparing your lecture; force yourself to define exactly what you want your students to learn that particular day. Decide upon your three points and fill in the details around them. If you attempt much more than that, you run the risk of losing the attention and interest of the class. The why has already been detailed (indeed, it's the one main point of this article) -- you don't want to have an experience like mine.
Your goal is not to prove to your students how much you know, but rather to teach them some of what you know. In your lectures, you can help synthesize a large amount of material and focus students on the most important aspects of the subject, and let the course text and supplementary readings fill in the rest.