Finally got around to reading this thread, and I agree, it's an interesting topic. Guess I'll take up the other side of the coin and play Devil's Advocate, shall I?
I disagree with Anoni. Although it may be true that people that have their path (or prize as used by the experiment) chosen for them are generally happier than people that choose their own, I still believe in Free Will. I will not deny that there are those that are more than happy subsisting and being told what to do (in my opinion, these people are little more than sheep and deserve to be led due to their unwillingness to take control of their own lives and take a stand, but that is a bit beside the point) there are those that
DO choose what they will do in their lives and accept the responsibility with great joy. Those that take control and gain responsibility (to reinforce the words you used in your first paragraph) accept the stress as a known byproduct and use that to fuel their continued pursuits. These people are the leaders of our society as they have the ability and have accepted the responsibility of not being the ones led around by others.
To move to your second paragraph, I feel that I've already stated my opinion on Free Will but I feel that your question about what defines us as human beings is a bit misplaced. Although I can't deny that genetics play a massive part in human behavior (if a single molecule is misplaced in the brain anything from death to retardation to personality modification is possible which is all controlled by a series of biochemical functions that all tie back to our Double-Helix DNA) and that our surroundings have no small part in our outlooks and attitudes, I feel that there is a third part: the Soul.
You make the comical statement "You're on the internet right now" which must be true as I have read your post and responded (troll) but then proceed to ask the question "why are you on the internet?" You state that I am on the internet because of my genetics and my society dictates that I do what makes me happy and that it is something I should do (to extrapolate from your given data). I would like to think that I am on the internet because I choose to do so.
I have accepted responsibilities and duties such as to keep my mental state within a healthy level so that I can be happy and productive, to moderate this forum to the best of my abilities, to better myself and become as close to perfect as I can as Otebon Albrecht, and because I have those responsibilities I have the ability to choose to follow them. So I find myself on the internet reading and responding to your post.
I agree with your last statement to a degree, but overall I must say I disagree. I cannot, in good consciousness, say that Free Will is the penultimate deciding factor in sentient life. That statement is simply too easy to refute with items like the Fight or Flight Response or involuntary actions. However, I believe that Free Will is something that we, as sentient life, have the ability to use at our discretion.
If our genetics and environment were the only, or overriding powerful, forces that controlled us, then I must pose the question why do people choose to do themselves harm? Their genetics is centered around the continuation of their species and the protection of their body and society, even throughout history, has rarely had a more than disgusted outlook on self harm of any sort. Although the probability that everyone will suddenly stampede the streets wearing nothing but a smile is infinitely minuscule, the probability that someone will do something to hurt themself or do something that will damage their futures for no better reason than "just because" is fairly high. To try and get away from the more sobering possibilities, focus on people that acquire body jewelry in highly visible areas like the tongue or nose? How about those that split their tongues so that they look like serpents? That pain is for no better reason that it looks "cool" by a minority of a society and such a deviation from the norm will probably injure their chances later in life.
To return to the original topic, of choices making people happy or unhappy, I feel that it isn't a question about whether or not
choices make people happy or not. I feel that the true question is the
number of choices that make people unhappy. Our world, especially in the "West" is growing exponentially. Every day a whole new series of choices and responsibilities is being thrown at every one of us as fast as technology can make them and people can try to sell them. The reason many may feel depressed due to freedoms is that they are being overwhelmed and find it easier to just let themselves be led. It's an ugly cycle as more and more responsibilities are offered and soon even the most stalwart defenders of Free Will will begin to feel the toll of choice.
To be offered a choice is freedom, in my opinion. To give me a choice is to offer me happiness as it lets me pick my own path. However, if you bombard me with choices and it becomes just too much... therein lies the problem.
Sorry if I sound aggressive in this... I kinda got into it and loved to get back into the realm of debate. Sorry Anoni!