Would you please critique and rate the essay out of 6?

[Freeland]

[LazarusRises]

I'm not going to attempt to rate it, but it doesn't look like you actually answered the prompt. That's probably the single most important element of these essays: read the prompt carefully, and respond only to the task you are given. 

They didn't ask you to tear apart the theory presented in the paragraph, they asked you to propose an alternate explanation for the facts presented. You do this somewhat in your second paragraph, but then return to dismantling the experiment's methodology in paragraphs 3 and 4. I would instead expand on your theory of cortisol levels being related to age rather than birth order, which is interesting. What about the fact that the prompt states "firstborn infant monkeys," implying that infant monkeys of different birth orders were also tested? Note also that the prompt doesn't request that you solve the issue or correct the experiment, only that you propose another possible reason for the data shown. Your age theory doesn't have to be right, you just have to show that you carefully considered it. 

One more thing to be aware of: the original paragraph doesn't ever actually state that the scientists believe birth order is related to cortisol production, only that firstborn monkeys have higher cortisol levels, as do first-time mothers. To state that "the fundamental premise of this letter is flawed" is an exaggeration, as that fundamental premise is simply the existence of higher cortisol levels in certain types of rhesus monkeys. 

Your argument is cogent, but it doesn't answer the prompt, and you'd lose points for that. When writing these essays, be sure you dissect the prompt itself to understand the specific task assigned to you. 

(Note: 6.0 AW score and a few years of editorial experience.)