In a recent set of posts there was a discussion about evolutionary psychology and how it can be used to explain various behaviors.  However, one of the fundamental challenges raised is whether the references to biological phenomenon are, in fact, settled issues and whether they should serve as a basis for drawing additional conclusions.  As a result, I wanted to focus on one particular paragraph from one paper to illustrate the problem.
"However, costly signaling theory is only beginning to influence thinking in psychology (Miller, 2000). The classic example of a costly signal is the peacock’s tail, whereby the quality of the tail—its size, color, luminosity, and symmetry—serves as an honest signal of the quality of the peacock’s genes to potential mates. A high-quality tail is costly to have because it takes much metabolic energy and esources to grow and maintain such a resplendent ornament, which is useless and even detrimental in other aspects of a peacock’s life; a high-quality tail is an honest signal of good genes because only those peacocks who are in good health and who have the traits required to survive and acquire abundant supplies of food can afford to waste their energy and resources to grow and maintain this showy and nutritionally costly ornament (Loyau, Saint Jalme, Cagniant,&Sorci, 2005; Møller&Petrie, 2002)."

Blatant Benevolence and Conspicuous Consumption: When Romantic Motives Elicit Strategic Costly Signals
Here we have a prime example of making a scientific assumption with little or no evidence for the actual facts.  Notice how the reference begins by citing a "classic example" despite the fact that there is nothing "classic" about it.  It simply suggests that this is a settled issue and therefore beyond questioning.

The basis for this statement is rooted in the idea that the peacock's tail 1, because of its extravagance, is a costly feature that will negatively impact male fitness and consequently its display is an "honest signal" to females regarding the male's genetic superiority.  The first problem is whether this is actually "costly".

To my knowledge there hasn't been a single study that actually relates peacock mortality to their tails.  In other words, if the tail is a handicap, then there must be some data which shows a higher vulnerability to predation than for those without it.  In the quote it is referred to as being metabolically "costly", although this is a value judgement and has no merit.  Unless it can be demonstrated to materially impede the peacock's ability to survive, by incurring a higher caloric requirement and hence cost, it is a meaningless statement.  More problematic, is the point that peacocks routinely molt and do not carry this tail with them for well over half their lives.  

The point here isn't that the peacock's tail isn't the product of sexual selection, but rather it is that the interpretation of its costs and benefits is entirely extrapolated without data, often based on little more than our own perceptions.  One thing that is ultimately true about sexual selection, is that for any trait to manifest in a population, it must be more beneficial than any counter-selection that could incur which impacts fitness.  Therefore, we need to rethink the "cost" of the peacock's tail since it clearly has little apparent impact on the ability for peacocks to survive year to year.  There is a paper that attempts to correlate the number of tail feathers to age, but it simply isn't known how this relates to sexual selection in any meaningful way.
"Such a choosy female would therefore mate with a peacock who has demonstrated his genetic quality by avoiding death by predation, disease and environmental extremes.  Whether peahens can and do assess numbers of ocelli remains to be demonstrated."
This also creates a secondary problem because it indicates that the tail cannot be a reliable indicator of the peacock's ability to survive predators, since it only grows during the mating season and does not reflect the actual history of the animal.  It is also problematic that the peacock will fan his tail when threatened, since this indicates exactly the opposite behavior that one would expect from an animal with such a "liability".

Another point made is that because of this "cost", the peacock generates an "honest signal" regarding genetic health.  
"Moller and Petrie took blood samples from male Blue Peafowl (Pavo cristatus) and recorded the numbers of B- and T-cells, and also measured the peacocks' tails and counted the number of eye spots. They discovered that the condition and length of the peacock's tail was related to the production of B-cells, and the size of the eye spots to T-cell production."
So, it appears that the tail may signal a healthy immune system, although it says nothing about the overall genetic quality nor survival capability of the peacock itself.  This certainly suggests that some information is being conveyed by the tail which is of importance during the mating rituals and acts as a "signaling" mechanism.  However, this does not support the claims made in the original quote which indicate "good genes" and the ability to acquire abundant supplies of food.  

One final problem remains.
"Combined with previous results, our findings indicate that the peacock's tail (1) is not the universal target of female choice, (2) shows small variance among males across populations and (3) based on current physiological knowledge, does not appear to reliably reflect the male condition."
In this study, it doesn't appear that the size of the tail actually influences female breeding behavior, at least not alone.
"A seven-year study of feral Indian peahens and peacocks in Japan [by Mariko Takahashi et al.] titled “Peahens Do Not Prefer Peacocks with More Elaborate Tails” notes that these peahens disregard plumage in making their mating choices. The study further observes that variations in male plumage do not reflect anything in particular about the male’s physiological condition. Most strikingly, this 2008 study finds that a high level of female hormones dulls a bird’s coloration, suggesting there has been selection on females with dull plumage. These scientists propose that both sexes started brightly colored. Females evolved dull coloration, which increases survival chances for this ground-nesting bird, since there is little male involvement in parenting and since the female is vulnerable to predation while incubating eggs on the ground. (The scientists also reviewed the literature on peacocks, found it contradictory in terms of supporting or refuting sexual-selection theory except in the sexual-selection field, and suggested that researchers take care to publish negative as well as positive results.)"
In the end, we are left with a "classic example" which turns out to not be classic at all.  In fact, the truth is that much of what we use to explain such behaviors is based on our own quaint notions of how we think this works, instead of building on detailed knowledge that can truly be used as a reliable indicator of biological behavior.  Darwin's theory was successful because it was simple and general enough to provide true insight, in addition to being nuanced enough to allow for varied interpretations.  There needs to be a much more cautious approach in explaining biological phenomenon, lest we engage in building ever flimsier hypothesis on little more than "just-so" stories.
Note that the peacock's trail is actually a "train" of covert tail feathers that comes off the back of the animal. However, for our purposes we will use the term "tail".