Evidence that the Federation has a communist economy...
TNG, "The Last Outpost"
DATA: The Ferengi are... well, the best description may be "traders."
PICARD: What kind of "traders"?
DATA: A more accurate comparison modern scholars have drawn from Earth history cites the ocean-going "Yankee Traders" of eighteenth and nineteenth century America, sir.
RIKER: From the history of my forebears? "Yankee Traders?"
DATA (nods): Who sail, in this case the galaxy, in search of mercantile and territorial opportunity.
RIKER: And are those scholars saying the Ferengi may not be too different from us?
DATA: Hardly sir. I believe the analogy refers to the worst quality of capitalists. The ferengi are believed to conduct their affairs of commerce on the ancient principle Caveat Emptor - "Let the buyer beware", sir.
The implication, here, is that Federation citizens do not "sail the galaxy in search of mercantile and territorial opportunity". They explore and they seek peaceful relations with other cultures, but they are not attempting to expand their territory, nor are they seeking advantageous trade opportunities.
TNG, "The Last Outpost"
LETEK: And there is even more! We can prove the hu-mans are destroyers of legal commerce ... also that they selfishly withhold vital technology from backward worlds...
MORDOC: And necessary defensive weapons, too. We Ferengi are now challenge this hu-man madness...
Riker and Data do not deny the Ferengi charges. IIRC, they admit that the Ferengi charges are accurate, they simply don't think they have done anything wrong. Whether you think they've done anything wrong or not, the implication is that "legal commerce" within Federation territory is restricted.
TNG, "The Neutral Zone"
RALPH: I demand to know the cost of anything you do before the procedure is approved.
BEVERLY: I have no idea what you're talking about.
The Federation has operated under a socialized medical system for so long that the very idea of paying for medical treatment is foreign to Dr. Crusher. This is not direct evidence of a communist system, but it is
consistent with one.
Mike's database didn't have a direct quote, but when Ralph asked for access to a communication system so he could check on his investments, Picard was as befuddled by the idea of a portfolio as Beverly was by the idea of paying for medical treatment. Lack of an investment market
is evidence of a communist economy.
TNG, "The Neutral Zone"
RALPH: And then what will happen to us? There's no trace of my money -- my office is gone -- what will I do? How will I live?
PICARD: This is the twenty-fourth century. Those material needs no longer exist.
Apparently, money isn't
necessary for a Federation citizen. Ralph and Picard are talking about the very concept of wealth here, not physical currency. Removing the need to accumulate money in order to live is a basic communist goal.
TNG, "Manhunt"
WORF: Captain... we're being hailed by a small transport vessel just coming into range.
TROI: Oh, my God!
PICARD: What is the problem?
TROI: What can she be doing here?
...
TRANSPORT PILOT: Starship Enterprise, come in!
RIKER: We have you on our viewer, Pilot.
TRANSPORT PILOT: Enterprise... I have a passenger -- a V.I.P. passenger -- who I'm ordered to --
MRS. TROI: Let me talk to them, Pilot. I'm much more articulate.
TROI: Mother!
DATA: Captain, we are now receiving Starfleet orders granting a Lwaxana....
MRS. TROI: Lwaxana Troi, daughter of the Fifth House, Holder of the Sacred Chalice of Riix, heir to the Holy Rings of Betazed.
DATA: ... full ambassadorial status, sir.
The Federation Ambassador from Betazed doesn't have her own personal transport or even a dedicated state transport. Owning a transport can't be all that expensive (Quark operates one off the proceeds of his bar, after all), so this is evidence that interstellar vehicles are tightly restricted in the Federation.
TNG, "Survivors"
TROI: Captain, there are eleven thousand inhabitants in the colony. At this range I should be feeling something.... I'm not.
...
DATA: Sensors are scanning ninety degrees of longitude as we orbit. I am detecting no artificial structures, no vegetation, no bodies of water...
WORF: Lifeform readings are negative.
TROI: Could the colonists have escaped?
DATA: That is unlikely. Rana Four possessed no interstellar spacecraft.
A colony with a population of 11,000 has
no interstellar vehicles. It's difficult to imagine why this colony wouldn't even have one interstellar shuttle unless ownership of such vehicles is strictly limited in the Federation.
TNG, "The Price"
RIKER: Our proposal includes technology that will enhance your food production, your mining capabilities, and space travel. We offer scholars and scientists to investigate the wormhole's potential. And perhaps most critical of all, we offer Starfleet. The security of this passage must never be compromised. We offer protection of the wormhole in perpetuity.
What's noticeably absent from Riker's proposal is an offer of
wealth. A capitalist offer would probably include economic incentives.
TNG, "The High Ground"
PICARD VO: Captain's log, Stardate 43510.7. The Enterprise has put in at Rutia Four to deliver medical supplies following an outbreak of violent protests. Although non-aligned, the planet has enjoyed a long trading relationship with the Federation.
The Federation does engage in trade, but that's not terribly surprising. The Soviet Union also engaged in trade to obtain resources and manufactured goods, but the
terms under which they conducted trade were much different from those of a capitalist economy like the US. In the US, private corporations trade for profit; in the USSR, the
government traded to obtain resources.
TNG, "The High Ground"
ALEXANA: Perhaps if we found ourselves in possession of some of that advanced Federation weaponry of yours it would shift the balance of power back to our favor.
PICARD: That we cannot do for you.
ALEXANA: No, of course not.
As Mike notes, the Ferengi apparently can't supply the weapons that the Rutians want, either, even though they would clearly be
willing to supply them (see quotes from "The Last Outpost").
TNG, "Allegiance"
PICARD: I found it unlikely that a first-year cadet would know of the Enterprise's visit to Mintaka Three... so I tested you. Starfleet has classified the Cor Caroli Five plague "secret." No cadet would have knowledge of the incident.
The ability to keep this plague outbreak secret from the general population is clearly evidence that the Federation is able and willing to restrict interplanetary communication.
TNG, "Brothers"
SOONG: And what's so important about the past? People needed money, they got sick. Why tie yourself to that?
Yet another claim that people no longer need money; such a claim would be ludicrous in a capitalist economy.
TNG, "Family"
(looking out over the vineyards)
LOUIS: One man's idea of paradise.
PICARD: Two men. Robert's. And my father's.
LOUIS: Never did I know anyone less interested in grapes than you, Jean-Luc.
PICARD: No, I was interested, Louis. And I was proud that my family helped to preserve the traditions. But I did not feel bound by those traditions as they seemed to be.
Maintaining the vinyard is described as a matter of tradition, not business. As Mike notes, Picard never speaks to his brother about sales or profits.
TNG, "Time's Arrow Part 2"
TROI: Poverty was eliminated a long time ago. And a lot of things disappeared with it: hopelessness, despair, cruelty, war ...
If Troi is to be believed, poverty isn't just rare in the Federation: it's
gone. This is a very unrealistic situation for a capitalist economy, but a fundamental goal of a communist economy.
TNG, "Firstborn"
QUARK: How could I forget the only man ever to win triple down dabo at one of my tables?
RIKER: And how could I forget that you didn't have enough latinum to cover my winnings?
QUARK: I thought I explained that my brother had misplaced the key to the safe. Besides, those vouchers I gave you are every bit as good as latinum.
RIKER: Not exactly. You can spend latinum just about anywhere. Those vouchers are only good at your bar.
Despite claims that Federation citizens are no longer motivated by money, Riker would still have preferred to have Quark pay his gambling winnings in something more liquid than bar vouchers. Clearly there are things that a Federation citizen
can buy with the right "currency", but these are presumably luxury items. After all, Federation citizens don't
need money, right?
TNG, "Firstborn"
RIKER: Where's this... Corvallen now? I have orders to acquire some magnesite.
YRIDIAN: He is gone ... Where? I don't know.
RIKER: Wouldn't you be interested in selling me the ore you're carrying?
YRIDIAN: No, I have a buyer.
RIKER: You haven't heard my offer.
RIKER: Half a gram of Anjoran bio-mimetic gel.
YRIDIAN: Done.
Riker arranges to buy ore from an Yridian, but he doesn't offer Federation currency. Instead he barters with some kind of valuable organic material in his possession.
Despite Alyeska's protests to the contrary, ST:TNG is full of indications that the Federation has a communist economy. Further analysis of DS9 and Voyager will have to wait for better on-line resources.
"This is supposed to be a happy occasion... Let's not bicker and argue about who killed who."
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"Nothing of consequence happened today. " -- Diary of King George III, July 4, 1776
"This is not bad; this is a conspiracy to remove happiness from existence. It seeks to wrap its hedgehog hand around the still beating heart of the personification of good and squeeze until it is stilled."
-- Chuck Sonnenburg on Voyager's "Elogium"