People's behavior is largely determined 
by forces not of their own making.

Human behaviors are interesting in that they can either be spontaneous or driven by external forces. Opinions vary greatly regarding the importance of these factors. The statement suggests that our behaviors are largely determined by external forces rather than our own qualities. While it may be true that external forces can greatly shape our behaviors, from my perspective, our own making also plays a role that cannot be overlooked. What is more, since external and internal forces have distinct natures and mechanisms to affect human behaviors, any attempt to claim one is more influential than the other is overly simplistic.

making (問題等)自己造成的;咎由自取
If you say that something such as a problem you have is of your own making, you mean you have caused or created it yourself.

External forces can play a critical role in regulating human behavior, because human beings are social and respond to external stimuli. Therefore, social norms determined to a large extent what can be or cannot be done. The reason wht human are a social species can be traced back to ancient times, when individuals are vulnerable to nature's elements. When they acted collectively, however, human became much stronger which substantially increased their chance of survival. Over time, being a part of a group of tribe became a survival advantage, with banishment often equaling demise. In this sense, one would face the risk of perishing alone if he or she defied social norms. Even though modern tools have drastically increased the odds of survival, we are still emotionlly attached to society, the modern tribe we belong to, and yearn for social recognition. Some of our basic motivations, such as aspiring to become wealthy, beautiful, intelligent, or powerful, can be explained by this desire. This yearning also prevents us from behaving outside of society acceptable norms.

stimuli n.促進(jìn)因素;激勵(lì)因素;刺激物;(使生物產(chǎn)生反應(yīng)的)刺激
with banishment often equaling demise 放逐常常等于死亡
perish 死亡;暴死;喪失;湮滅;毀滅;(使橡膠等)老化贵涵，脆裂
yearn 渴求 渴望

That being said, I must point out that humans are mentally capable of making choices of their own will and are physically capable of executing these choices.This is particularly true when our choices involve little, if any, interaction with the external environment. In this case, our choices do not conflict with societal guidelines. If one wants to pick up a restaurant for dinner in the Midtown of New York City, for example, there are many choices at his or her disposal: French, Asian, American, and Mexican and so on. This choice is not determined by external factors, but rather depends on a person's own will. Examples like these suggest that when our behavior has little to do with our social environment, we can and will make choices that reflect our own volition.

That being said 也就是說
societal adj.社會(huì)的;關(guān)于社會(huì)的
at his or her disposal 由他或她決定
volition n.意志力;自愿選擇;自行決斷

From the discussion above, it is clear that both external and internal forces are capable of determining human behavior, and most of the time we cannot disregard either's influence. But neither can we compare their importance. In some cases, external forces determine what needs to be done but we ultimately decide how to behave while adhering to social norms. For instance, a scientist will be forced to secure funding if his or her laboratory faces financial hardship. Though this necessity is solely determined by external forces, how best to secure those funds is at the scientist's discretion: he or she can seek federal support or form a collaborative relationship with industry. Either way, his or her action is not influenced by external factors. How external and internal forces can jointly shape human behavior and stipulate rules which must be followed but solutions are based on internal factors. This makes a difficult to argue that external forces play a more siginificant role in determining people's behavior.

discretion n.自行決定的自由;自行決定權(quán);謹(jǐn)慎;慎重;審慎

To sum up, I have demonstrated that both external and internal force are capable of determining people's behavior. Given the different mechanisms through which external and internal forces shape how we behave, conclusion drawn form a hasty comparison would certainly be oversimplified.