So to jump right in, what is mood?
To dig deeper, there is a minimum amount of certain resources the body needs to function, such as energy, nutrients, water, and even social interactions and there is the amount of each available to you.
In psychological terms, the resources we need are things we don't have to learn to like, which are referred to as unconditioned reinforcers(URs). These include the items mentioned above, along with things such as sugar or sex. There are also conditioned reinforcers(CRs), which are simply things you can detect that predict the eventual receipt of unconditioned reinforcers. For example, you aren't born appreciating dollar bills, but learn to like them due to the things you can purchase with them. I just introduce these terms to readers less familiar with psychology, as they may appear in later posts. By the way, psychologists refer to anything we can detect as "stimuli" or a "stimulus" in the singular.
As a brief note, notice that in the definition given above, I even include past intake. This is the recognition that not only can past pleasureable or displeasurable events be remembered, but it is actually a longer term average of mood levels that, along with present and expected future intake of peasure or pain, bias current reactions to events that temporarily change mood. For example, someone with long term depression can occasionally smile, but their longer term average mood state llimits the amount of pleasure they can experience now, while making displeasurable experiences more severe.
Neurobiologically(skip this, unless interested), this is facilitated by the death of cells in the hippocampus in the brain, which are required to lower levels of stress hormones in the body after displeasurable events. With higher average mood levels over enough time, these cells regenerate. This just gives an idea of the kind of motivational inertia low average moods demand, and is very much related to depression.
Mood shapes how we value things. The relative amount of value we place on pleasurable experiences are less when mood is lower compared to the value placed on displeasurable ones. In the absolute sense however, lower mood leads to greater values of both pleasure and displeasure, but the latter to a greater degree(change at a greater rate, with mood decreases). Similary, higher moods lead to the opposite case.
The basic concept is very simple. The more of something you have, generally the less of that something you want or need. And of course, the reverse is true. To use a familiar example, if you are eating potato chips, the hungrier you are, the less likely you are to share chips. But no matter how hungry you are, if you have enough chips, you will be willing to share them. The same is true with money, of course, as first outlined by Daniel Bernoulli quite some time ago.
I will now briefly present a curve, which has the same shape as the one in the Bernoulli link above, to clarify the way mood affects how we value units of pleasure and displeasure(gains and losses):
Ignore the equation in this graph, unless interested, in which case you can learn more here and here.
You notice above the now familar term "reinforcement",a nd then and "reponse rate." Perhaps not surprisingly, net reinforcement(again, pleasure - displeasure), should determine how much of our resources, such as time, energy, etc., we invest in a given behavior. Hence, you see the relationship between a response rate and reinforcement rate, the former of which can also be considered simply as energy expenditure.
This sort of curve would also fit the relationship between the subjective values placed on gains and losses(pleasure and displeasure) and those on objective gains and losses, the latter if not for the influence of mood. Replace "reinforcement rate" on the horizontal axis with "objective gains and loss values" and "response rate" with "subjective gains and loss values" on the vertical. Now consider any point on this curve to correspond to a specific mood level.
Whether you think about it or not, every behavior you engage in involves a prior calculation of expected gain - expected loss, in the objective sense. A gain will move mood up on the curve and a loss, down on the curve. So, the subjective difference between an expected gain and loss changes depending on the mood level. Moving up on the curve means that both gains and losses are valued less, and the difference is greater in favor of net gains, compared to objective gains and losses. Similarly, moving down on the curve means that both subjective gains and losses grow in value, but those of losses more quickly. .
So, mood is like a general currency in which all pleasure and displeasure is converted. You can refer to the net units of mood, or pleasure - displeasure, .as units of reinforcement, or use the terms economists use, such as utils or even units of demand. You can even just use "units of pleasure". It's not really imporant, unless trying to discuss these concepts with people in related fields.
To sum up, this is very similar to the income effect, in which the higher the monetary income, the less you value each additional dollar earned, all else being equal. For example, see the curve below:
Utility is the same as net pleasure for our purposes, or you can say "net motivation."
This final illustration makes it easy to consider some effects of changes in mood. When mood decreases, there is a shift in preference toward more "profitable" behaviors. These include the things that bring us the most pleasure, quickest. Good examples are sex, recreational drugs, and high calorie foods containing fat and sugar. It is perhaps anecdotally clear that when we feel down, we are more susceptible to sexual seduction, drinking alcohol, or eating comfort foods. This is just as a company that becomes less profitable begins to shift resources to more profitable operations, cutting or eliminating less profitable ones. There is generally a need for more income, or a higher rate of intake of resources. You may also realize that this explains why people who are depressed also have more trouble delaying gratification with implications for procrastination, among other difficulties. Perhaps it is also clear why negative emotional responses are more severe when mood is low.
Speaking of depression, the above now gives us a good model. Depression is a motivational trap in which as mood decreases, again, the subjective values of gains and losses both increase, but those of losses more quickly. Hence, the seeming confusion resulting when there is a greater desire or need to engage in certain behaviors, but less motivation to do so. This is seen particularly in many depressed patients, especially women, who may not seek sex as much when depressed, but may be more likely to engage in it when offered with relatively less effort required. On the other hand, some become more aggressive when seeking sex, if the costs of doing so are sufficiently low. From a prescriptive point of view, this supports the need for psychotropic drugs to help pull patients out of this often self-feeding motivational trap. This should increase the ability of patients to then invest their time and other resources (including attention) in counseling for greater benefit.
Finally, why do we have mood and why does it work this way? Well, these mood-related effects help motivate us to conserve more of our resources the lower the availability of them becomes. Though it may seem maladaptive for the brain to work this way, and it is in the modern environment, in an environment of relative scarcity it makes a lot of sense. Such environments are those we inhabited within the stereotypical hunter-gatherer groups for our 200,000 year prehistory.
I hope I avoided getting too technical here, while offering enough information to explain this conception of mood. I will appreciate any feedback on my success in his sense.