CC0 is superior to any of the Creative Commons (CC) 4.0 licenses, because CC0 represents a superior policy (public domain). But if you’re unable or unwilling to upgrade to CC0, the CC 4.0 licenses are a great improvement over the 3.0 licenses. The people who did the work, led by Diane Peters (who also led CC0), many CC affiliates (several of whom were also crucial in making CC0 a success), and Sarah Pearson and Kat Walsh, deserve much praise. Bravo!

Below read my idiosyncratic take on issues addressed and not addressed in the 4.0 licenses. If that sounds insufferable, but you want to know about details of the 4.0 licenses, skip to the excellent version 4 and license versions pages on the CC wiki. I don’t bother linking to sections of those pages pertinent to issues below, but if you want detailed background beyond my idiosyncratic take on each issue, it can be found there.

Any criticism I have of the 4.0 licenses concerns policy choices and is not a criticism of the work done or people involved, other than myself. I fully understand that the feasible choices were and are highly constrained by previous choices and conditions, including previous versions of the CC licenses, CC’s organizational history, users of CC licenses, and the overall states of knowledge commons and info regulation and CC’s various positions within these. I always want CC and other “open” organizations to take as pro-commons of a stance as possible, and generally judge what is possible to be further than that of the conventional wisdom of people who pay any attention to this scene. Sometimes I advocated for more substantial policy changes in the 4.0 licenses, though just as often I deemed such advocacy futile. At this point I should explain that I worked for CC until just after the 4.0 licenses process started, and have consulted a bit on 4.0 licenses issues since then as a “fellow”. Not many people were in a better position to influence the 4.0 licenses, so any criticisms I have are due to my failure to convince, or perhaps incorrect decision to not try in some cases. As I’ve always noted on this blog, I don’t represent any organization here.

Desiderata

Pro-commons? As opposed to what? The title of the CC blog post announcing the formal beginning of work on the new licenses:

Copyright Experts Discuss CC License Version 4.0 at the Global Summit

My personal blog post:

Commons experts to develop version 4.0 of the CC licenses

The expertise that CC and similar organizations ought to bring to the world is commons coordination. There are many copyright experts in the world, and understanding public copyright licenses, and drafting more, are no great intellectual challenges. The copyright expertise needed to do so ought be purely instrumental, serving the purpose of commons coordination. Or so I think.

Throughout CC’s existence, it has presented itself, and been perceived as, to varying extents, an organization which provides tools for copyright holders to exercise their copyrights, and an organization which provides tools for building a commons. (What it does beyond providing tools adds another dimension, not unrelated to “copyright choice” vs. “commons coordination”; there’s some discussion of these issues in a video included in my personal post above.)

I won’t explain in this post, but I think the trend through most of CC’s history has been very slow movement in the “commons coordination” direction, and the explicit objectives of the 4.0 versioning process fit that crawl.

“Commons coordination” does not directly imply the usual free/open vs. proprietary/closed dichotomy. I think it does mostly fall out that way, in small part due to “license interoperability” practicalities, but probably mostly because I think the ideal universal copyregulation policy corresponds to the non-discriminatory commons that “free/open” terms and communities carve out on a small scale, including the pro-sharing policy that copyleft prototypes, and excluding any role for knowledge enclosure, monopoly, property, etc. But it is certainly possible, indeed usual, to advocate for a mixed regime (I enjoy the relatively new term “semicommons”, but if you wish to see it everywhere, try every non-demagogic call for “balance”), in which case [semi]commons tools reserving substantial exclusivity (e.g., “commercial use”) make perfect sense for [semi]commons coordination.

Continuing to ignore the usual [non-]open dichotomy, I think there still are a number of broad criteria for would-be stewards of any new commons coordinating license (and make no mistake, a new version of a license is a new license; CC introduced 6 new licenses with 4.0) to consider carefully, and which inform my commentary below:

• Differentiation: does the new license implement some policy not currently available in existing licenses, or at least offer a great improvement in implementation (not to provide excuses for new licenses, but the legal text is just one part of implementation; also consider branding/positioning, understandability, and stewardship) of policy already available?
• Permissions: does the new license grant all permissions needed to realize its policy objective?
• Regulation: how does the license’s policy objective model regulation that ought be adopted at a wider scale, e.g., how does it align with usual “user rights” and “copyright reform” proposals?
• Interoperability: is the new license maximally compatible with existing licenses, given the constraints of its policy objectives, and indeed, to the expense of its immediate policy objectives, given that incompatibility, non-interoperability, and proliferation must fragment and diminish the value of commons?
• Cross-domain impact: how does the license impact license interoperability and knowledge sharing across fields/domains/communities (e.g., software, data, hardware, “content”, research, government, education, culture…)? Does it further silo existing domains, a tragedy given the paucity of knowledge about governing commons in the world, or facilitate sharing and collaboration across domains?

Several of these are merely a matter of good product design and targeting, and would also apply to an organization that really had a primary goal of offering copyright holders additional choices the organization deems are under-provided. I suspect there is plenty of room for innovation in “copyright choice” tools, but I won’t say more in this post, as such have little to do with commons, and whatever CC’s history of copyright choice rhetoric and offering a gaggle of choices, creating such tools is distant from its immediate expertise (other than just knowing lots about copyright) and light years from much of its extended community.

Why bother?

Apart from amusing myself and a few others, why this writeup? The CC 4.0 licenses won’t change, and hopefully there won’t be CC 4.1 or 4.5 or 5.0 licenses for many years. Longevity was an explicit goal for 4.0 (cf. 1.0: 17 months, 2.0: 12 months; 2.5: 20 months; 3.0: 81 months). Still, some of the issues covered here may be interesting to people choosing to use one of the CC 4.0 licenses, and people creating other licenses. Although nobody wants more licenses, often called license proliferation, as an end in itself, many more licenses is the long term trend, of which the entire history of CC is just a part. Further, more licenses can be a good, to the extent they are significantly different from and better than, and as compatible as possible with, existing licenses.

To be totally clear: many new licenses will be created and used over the next 10 years, intended for various domains. I would hope, some for all domains. Proliferators, take heed!

Development tools

A 4.0 wiki page and a bunch of pages under that were used to lay out objectives, issues and options for resolution, and link to drafts. Public discussion was on the cc-licenses list, with tangential debate pushed to cc-community. Drafts and changes from previous drafts were published as redlined word processor files. This all seems to have worked fairly well. I’d prefer drafts as plain text files in a git repository, and an issue tracker, in addition to a mailing list. But that’s a substantially different workflow, and word processor documents with track changes and inline comments do have advantages, not limited to lawyers being familiar with those tools.

100% wiki would also work, with different tradeoffs. In the future additional tools around source repositories, or wikis, or wikis in source repositories, will finally displace word processor documents, but the tools aren’t there yet. Or in the bad future, all licenses will be drafted in word processors in the cloud.

(If it seems that I’m leaving a a lot out, e.g., methodology for gathering requirements and feedback, in-person and teleconferences, etc., I merely have nothing remotely interesting to say, and used “tools” rather than “process” to narrow scope intentionally.)

Internationalization

The 4.0 licenses were drafted to be jurisdiction neutral, and there will be official, equivalent, verbatim language translations of the licenses (the same as CC0, though I don’t think any translations have been made final yet). Legal “porting” to individual jurisdictions is not completely ruled out, but I hope there will be none. This is a wholly positive outcome, and probably the most impactful change for CC itself (already playing out over the past few years, e.g., in terms of scope and composition of CC affiliates), though it is of small direct consequence to most users.

Now, will other license drafters and would-be drafters follow CC’s lead and stop with the vanity jurisdiction license proliferation already?

Databases

At least the EU, Mexico, Russia, and South Korea have created “database rights” (there have been attempts in other jurisdictions), copyright-like mechanisms for entities that assemble databases to persecute others who would extract or copy substantial portions of said databases. Stupid policies that should be abolished, copyright-like indeed.

Except for CC0 and some minor and inconsistent exceptions (certain within-EU jurisdiction “port” versions), CC licenses prior to 4.0 have not “covered” database rights. This means, modulo any implied license which may or may not be interpreted as existing, that a prior-to-4.0 (e.g., CC-BY-3.0) licensee using a database subject to database restrictions (when this occurs is a complicated question) would have permission granted by the licensor around copyright restrictions, but not around database restrictions. This is a pretty big fail, considering that the first job of a public license is to grant adequate permissions. Actual responses to this problem:

• Tell all database publishers to use CC0. I like this, because everyone should just use CC0. But, it is an inadequate response, as many will continue to use less permissive terms, often in the form of inadequate or incompatible licenses.
• Only waive or license database restrictions in “ports” of licenses to jurisdictions in which database restrictions exist. This is wholly inadequate, as in the CC scheme, porting involves tailoring the legal language of a license to a jurisdiction, but there’s no guarantee a licensor or licensee in such jurisdictions will be releasing or using databases under one of these ports, and in fact that’s often not the case.
• Have all licenses waive database restrictions. This sounds attractive, but is mostly confusing — it’s very hard to discern when only database and not copyright restrictions apply, such that a licensee could ignore a license’s conditions — and like “tell database publishers to use CC0” would just lead many to use different licenses that do purport to conditionally license database rights.
• Have all licenses grant permissions around database restrictions, under whatever conditions are present in the license, just like copyright.

I think the last is the right approach, and it’s the one taken with the CC 4.0 licenses, as well as by other licenses which would not exist but for CC 3.0 licenses not taking this approach. I’m even more pleased with their generality, because other copyright-like restrictions are to be expected (emphasis added):

Copyright and Similar Rights means copyright and/or similar rights closely related to copyright including, without limitation, performance, broadcast, sound recording, and Sui Generis Database Rights, without regard to how the rights are labeled or categorized. For purposes of this Public License, the rights specified in Section 2(b)(1)-(2) are not Copyright and Similar Rights.

The exclusions of 2(b)(1)-(2) are a mixed bag; see moral and personality rights, and patents below.

CC0 also includes a definition with some generality:

Copyright and Related Rights include, but are not limited to, the following:

1. the right to reproduce, adapt, distribute, perform,
   display, communicate, and translate a Work;
2. moral rights retained by the original author(s) and/or
   performer(s);
3. publicity and privacy rights pertaining to a person’s
   image or likeness depicted in a Work;
4. rights protecting against unfair competition in regards
   to a Work, subject to the limitations in paragraph 4(a),
   below;
5. rights protecting the extraction, dissemination, use and
   reuse of data in a Work;
6. database rights (such as those arising under Directive
   96/9/EC of the European Parliament and of the Council of 11
   March 1996 on the legal protection of databases, and under
   any national implementation thereof, including any amended
   or successor version of such directive); and
7. other similar, equivalent or corresponding rights
   throughout the world based on applicable law or treaty, and
   any national implementations thereof.

As does GPLv3:

“Copyright” also means copyright-like laws that apply to other kinds of works, such as semiconductor masks.

Do CC0 and CC 4.0 licenses cover semiconductor mask restrictions (best not to use for this purpose anyway, see patents)? Does GPLv3 cover database restrictions? I’d hope the answer is yes in each case, and if the answer is no or ambiguous, future licenses further improve on the generality of restrictions around which permissions are granted.

There is one risk in licensing everything possible, and culturally it seems, specifically in licensing database rights — the impression that licensee which do so ‘create obligations’ related to those rights. I find this an odd way to think of a conditional permission as the creation of an obligation, when the user’s situation without said permission is unambiguously worse, i.e., no permission. Further, this impression is a problem for non-maximally-permissive licenses around copyright, not only database or other copyright-like rights.

In my opinion the best a public license can do is to grant permissions (conditionally, if not a maximally permissive license) around restrictions with as much generality as possible, and expressly state that a license is not needed (and therefore conditions to not apply) if a user can ignore underlying restrictions for some other reason. Can the approach of CC version 4.0 licenses to the latter be improved?

For the avoidance of doubt, where Exceptions and Limitations apply to Your use, this Public License does not apply, and You do not need to comply with its terms and conditions.

These are all trivialities for license nerds. For publishers and users of databases: Data is free. Free the data!

Moral and personality rights

CC 4.0 licenses address them well:

Moral rights, such as the right of integrity, are not licensed under this Public License, nor are publicity, privacy, and/or other similar personality rights; however, to the extent possible, the Licensor waives and/or agrees not to assert any such rights held by the Licensor to the limited extent necessary to allow You to exercise the Licensed Rights, but not otherwise.

To understand just how well, CC 3.0 licenses say:

Except as otherwise agreed in writing by the Licensor or as may be otherwise permitted by applicable law, if You Reproduce, Distribute or Publicly Perform the Work either by itself or as part of any Adaptations or Collections, You must not distort, mutilate, modify or take other derogatory action in relation to the Work which would be prejudicial to the Original Author’s honor or reputation. Licensor agrees that in those jurisdictions (e.g. Japan), in which any exercise of the right granted in Section 3(b) of this License (the right to make Adaptations) would be deemed to be a distortion, mutilation, modification or other derogatory action prejudicial to the Original Author’s honor and reputation, the Licensor will waive or not assert, as appropriate, this Section, to the fullest extent permitted by the applicable national law, to enable You to reasonably exercise Your right under Section 3(b) of this License (right to make Adaptations) but not otherwise.

Patents and trademark

Prior versions were silent, CC 4.0 licenses state:

Patent and trademark rights are not licensed under this Public License.

Perhaps some potential licensor will be reassured, but I consider this unnecessary and slightly harmful, replicating the main deficiency of CC0. The explicit exclusion makes it harder to see an implied license. This is especially troublesome when CC licenses are used in fields in which patents can serve as a barrier. Software is one, for which CC has long disrecommended use of CC licenses largely because software is already well-covered by licenses with which CC licenses are mostly incompatible with; the explicit patent exclusion in the CC 4.0 licenses makes them even less suitable. Hardware design is another such field, but one with fragmented licensing, including use of CC licenses. CC should now explicitly disrecommend using CC licenses for hardware designs and declare CC-BY-SA-4.0 one-way compatible with GPLv3+ so that projects using one of the CC-BY-SA licenses for hardware designs have a clear path to a more appropriate license.

Patents of course can be licensed separately, and as I pointed out before regarding CC0, there could be curious arrangements for projects using such licenses with patent exclusions, such as only accepting contributions from Defensive Patent License users. But the better route for “open hardware” projects and the like to take advantage of this complementarity is to do both, that is use a copyright and related rights license that includes a patent peace clause, and join the DPL club.

DRM

CC 4.0 licenses:

The Licensor waives and/or agrees not to assert any right or authority to forbid You from making technical modifications necessary to exercise the Licensed Rights, including technical modifications necessary to circumvent Effective Technological Measures.

This is a nice addition, which had been previously suggested for CC 3.0 licenses and rejected — the concept copied from GPLv3 drafts at the time. I would have preferred to also remove the limited DRM prohibition in the CC licenses.

Attribution

The CC 4.0 licenses slightly streamline and clarify the substance of the attribution requirement, all to the good. The most important bit, itself only a slight streamlining and clarification of similar in previous versions:

You may satisfy the conditions in Section 3(a)(1) in any reasonable manner based on the medium, means, and context in which You Share the Licensed Material. For example, it may be reasonable to satisfy the conditions by providing a URI or hyperlink to a resource that includes the required information.

This pulls in the wild use from near zero to-the-letter compliance to fairly high.

I’m not fond of the requirement to remove attribution information if requested by the licensor, especially accurate information. I don’t know whether a licensor has ever made such a request, but that makes the clause only pointless rather than harmful. Not quite though, as it does make for a talking point.

NonCommercial

not primarily intended for or directed towards commercial advantage or private monetary compensation. For purposes of this Public License, the exchange of the Licensed Material for other material subject to Copyright and Similar Rights by digital file-sharing or similar means is NonCommercial provided there is no payment of monetary compensation in connection with the exchange.

Not intended to be a substantive change, but I’ll take it. I’d have preferred a probably more significantly narrowed definition and a re-branding so as to increase the range of and differentiation among the licenses that CC stewards. But at the beginning of the 4.0 licenses process, I expected no progress, so am not disappointed. Branding and other positioning changes could come post-launch, if anyone is so inclined.

I think the biggest failure of the range of licenses with an NC term (and there are many preceding CC) is not confusion and pollution of commons, very roughly the complaints of people who would like NC to have a more predictable meaning and those who think NC offers inadequate permissions, respectively, but lack of valuable use. Licenses with the NC term are certainly used for hundreds of millions of photos and web pages, and some (hundreds of?) thousands of songs, videos, and books, but few where either the licensor or the public gains significant value above what would have been achieved if the licensor had simply offered gratis access (i.e., put stuff on the web, which is incredibly valuable even with no permissions granted). As far as I know, NC licenses haven’t played a significant role in enabling (again, relative to gratis access) any disruptive product or policy, and their use by widely recognized artists and brands is negligible (cf. CC-BY-SA, which Wikipedia and other mass collaboration projects rely on to exist, and CC-BY and CC0, which are part of disruptive policy mandates).

CC is understandably somewhat stuck between free/open norms, which make licenses with the NC an embarrassment, and their numerically large but low value uses. A license steward or would-be steward that really believed a semicommons license regime could do much more would try to break out of this rut by doing a complete rethink of the product (or that part of the product line), probably resulting in something much more different from the current NC implementation than the mere definitional narrowing and rebranding that I started out preferring. This could be related to my commentary on innovation in “copyright choice” tools above; whether the two are really the same thing would be a subject for inquiry.

NoDerivatives

If there were licenses that should not have been brought to version 4.0, at least not under the CC brand, it would have been CC-BY-NC-ND and CC-BY-ND.

Instead, an express permission to make derivatives so long as they are not shared was added. This change makes so-called text/content/data mining of any work under any of the CC version 4.0 licenses unambiguously permitted, and makes ND stick out a tiny bit less as an aberration from the CC license suite modeling some moderate copyright reform baseline.

There are some costs to this approach: surprise that a “no derivatives” license permits derivatives, slight reduction in scope and differentiation among licenses that CC stewards, giving credence to ND licenses as acceptable for scholarship, and abetting the impression that text/content/data mining requires permission at all. The last is most worrisome, but (as with similar worries around licensing databases) can be turned into a positive to the extent CC and everyone knowledgeable emphasizes that you ought not and probably don’t need a license; we’re just making sure you have the freedoms around CC licensed works that you ought to have anyway, in case the info regulation regime gets even worse — but please, mine away.

ShareAlike

This is the most improved named (BY/NC/ND/SA) elements in CC 4.0 licenses, and the work is not done yet. But first, I wish it had been improved even more, by making more uses unambiguously “trigger” the SA provision. This has been done once, starting in 2.0:

For the avoidance of doubt, where the Work is a musical composition or sound recording, the synchronization of the Work in timed-relation with a moving image (“synching”) will be considered a Derivative Work for the purpose of this License.

The obvious next expansion would have been use of images (still or moving) in contextual relation to other material, eg illustrations used in a text. Without this expansion, CC-BY-SA and CC-BY-NC-SA are essentially identical to CC-BY and CC-BY-NC respectively for the vast majority of actual “reuse” instances. Such an expansion would have substantially increased the range of and differentiation among licenses that CC stewards. The main problem with such an expansion (apart from specifying it exactly) would be increasing the cost of incompatibility, where texts and images use different licenses. This problem would be mitigated by increasing compatibility among copyleft licenses (below), or could be eliminated by broadening the SA licensing requirement for uses triggered by expansion, eg any terms granting at least equivalent permissions, such that a CC-BY-SA illustration could still be used in a text licensed under CC-BY or CC0. Such an expansion did not make the cut, but I think together with aforementioned broadening of licensing requirements, such a modulation (neither strictly “stronger” nor “weaker”) would make for an interesting and heretofore unimplemented approach to copyleft, in some future license.

Apart from a subtle improvement that brings SA closer to a full “or later versions” license, and reflects usual practice and understanding (incidentally, “no sublicensing” in non-SA licenses remains pointless, is not to be found in most non-CC permissive licenses, and should not be replicated), the big improvements in CC 4.0 licenses with the SA element are the addition of the potential for one-way compatibility to CC-BY-SA, adding the same compatibility mechanism to CC-BY-NC-SA, and discussions with stewards of potentially compatible licenses which make the realization of compatibility more likely. (I would have included a variation on the more complex but in my view elegant and politically advisable mechanism introduced in MPL 2.0, which allows for continued use under the donor compatible license as long as possible. Nobody demanded such, so not adding the complexity was perhaps a good thing.)

I hope that in 2014 CC-BY-SA-4.0 will be declared bilaterally compatible with the Free Art License 1.3, or if a new FAL version is required, it is being worked on, with achieving bilateral compatibility as a hard requirement, and more importantly, that CC-BY-SA-4.0 is declared one-way compatible (as a donor) with GPLv3+. An immediate step toward those ends will be finalizing an additional statement of intent regarding the stewardship of licenses with the ShareAlike element.

Though I’ll be surprised if any license appears as a candidate for compatibility with CC-BY-NC-SA-4.0, adding the mechanism to that license is a good thing: as a matter of general license stewardship, reducing the barriers to someone else creating a better NC license (see above), and keeping “porting” completely outside the 4.0 license texts (hopefully there will be no porting, but if there is any, compatibility with the international versions in licenses with the SA element would be exclusively via the compatibility mechanism used for any potentially compatible license).

Tech

All license clauses have id attributes, allowing direct linking to a particular clause. These direct links are used for references within the licenses. These are big usability improvements.

I would have liked to see an expansive “tech” (including to some extent design) effort synchronized with the 4.0 licenses, from the practical (e.g., a canonical format for license texts, from which HTML, plain text, and others are generated; that may be HTML, but the current license HTML is inadequate for the task) to the impractical (except for increasing CC’s reputation, e.g., investigating whether any semantic annotation and structure, preferably building on existing research, would be useful, in theory, for the license texts, and possibly even a practical aid to translation), to testing further upgrades to the ‘legal user interface’ constituted by the license texts and “deed” summaries (e.g., combining these), to just bringing various CC tooling and documentation up to date with RDFa 1.1 Lite. But, some of these things could be done post-launch if anyone is so inclined, and my understanding is that CC has only a single technology person on staff, dedicated to creating other products, and most importantly, the ability to directly link to any license clause probably has more practical benefits than anything on my wishlist.

Readability

One of the best things about the CC 4.0 licenses is their increased understandability. This is corroborated by crude automated readability metrics below, but I suspect these do not adequately characterize the improvement, for they include three paragraphs of explanatory text not present in previous versions, probably don’t fully reflect the improvement of splitting hairball paragraphs into lists, and have no mechanism for accounting for how the improved usability of linking to individual clauses contributes to understandability.

CC-BY-NC-SA (the license with the most stuff in it, usually used as a drafting template for others) from version 1.0 through 4.0, including 4.0 drafts (lower numbers indicate better readability, except in the case of Flesch; Chars/(Flesch>=1) is my gross metric for how painful it is to read a document; see license automated readability metrics for an explanation):

SHA1	License	Characters	Kincaid	ARI	Coleman-Liau	Fog	Lix	SMOG	Flesch	Chars/(Flesch>=1)
39b2ef67be9e5b4e743e5269a31ad1691515eede	CC-BY-NC-SA-1.0	10228	13.3	16.3	14.2	17.0	59.7	14.2	48.4	211
5800ac2d32e35ace035cdcae693423cd9ff5bb6f	CC-BY-NC-SA-2.0	11927	13.3	16.2	14.7	17.1	60.0	14.4	47.0	253
e5f44c2df6b1391d1ddb6efb2db6f90670e4ae67	CC-BY-NC-SA-2.5	12013	13.1	16.0	14.6	16.9	59.6	14.2	47.7	251
a63b7e81e7b9e30df5d253aed1d2991af47992df	CC-BY-NC-SA-3.0	17134	16.4	19.7	14.2	20.6	67.0	16.3	38.8	441
8b36c30ed0510d9ca9c69a2ef826b9fd52992474	by-nc-sa-4.0d1	12465	13.0	15.0	14.9	16.3	57.4	14.0	43.9	283
4a87c7af5cde7729e2e456ee0e8958f8632e3005	by-nc-sa-4.0d2	11583	13.1	14.8	14.2	16.8	56.2	14.4	44.7	259
bb6f239f7b39343d62440bff00de24da2b3d256f	by-nc-sa-4.0d3	14422	14.1	15.8	15.1	18.2	61.0	15.4	38.6	373
cf5629ae38a745f4f9eca429f7b26af2e71eb109	by-nc-sa-4.0d4	14635	13.8	15.6	15.5	17.8	60.2	15.2	38.6	379
a5e1b9829fd287cbe255df71eb9a5aad7fb19dbc	by-nc-sa-4.0d4v2	14808	14.0	15.8	15.5	18.0	60.6	15.2	38.1	388
887f9a5da675cf681421eab3ac6d61f82cf34971	CC-BY-NC-SA-4.0	14577	13.1	14.7	15.7	17.1	58.6	14.7	40.1	363

Versions 1.0 through 4.0 of each of the six CC licenses brought to version 4.0, and CC0:

SHA1	License	Characters	Kincaid	ARI	Coleman-Liau	Fog	Lix	SMOG	Flesch	Chars/(Flesch>=1)
74286ae0dfea38c489437bf659b209737945145c	CC0-1.0	5116	16.2	19.5	15.0	19.5	66.3	15.6	36.8	139
c766cc6d5e63277e46a3d83c6254e3528082587b	CC-BY-1.0	8867	12.6	15.5	14.1	16.4	57.8	13.8	51.3	172
bf23729bec8ffd0de4d319fb33395c595c5c762b	CC-BY-2.0	9781	12.1	14.9	14.3	16.1	56.7	13.7	51.9	188
024bb6d37d0a17624cf532bd14fbd42e15c5a963	CC-BY-2.5	9867	11.9	14.7	14.2	15.8	56.3	13.6	52.6	187
20dc61b94cfe1f4ba5814b340095b4c3fa23e801	CC-BY-3.0	14956	16.1	19.4	14.1	20.4	66.1	16.2	40.0	373
00b29551deee9ced874ffb9d29379b92f1487045	CC-BY-4.0	13003	13.0	14.5	15.4	16.9	57.9	14.6	41.1	316
e0c4b13ec5f9b5702d2e8b88d98b803e07d65cf8	CC-BY-NC-1.0	9313	13.2	16.2	14.3	17.0	59.3	14.1	49.3	188
970421995789d2e8189bb12071ab838a3fcf2a1a	CC-BY-NC-2.0	10635	13.1	16.1	14.6	17.2	59.5	14.4	48.1	221
08773bb9bc13959c6f00fd49fcc081d69bda2744	CC-BY-NC-2.5	10721	12.9	15.8	14.5	16.9	59.0	14.2	48.9	219
9639556280637272ace081949f2a95f9153c0461	CC-BY-NC-3.0	15732	16.5	19.9	14.1	20.8	67.2	16.4	38.7	406
afcbb9791897e1e2f949d9d56ba64164746e0828	CC-BY-NC-4.0	13520	13.2	14.8	15.6	17.2	58.6	14.8	39.8	339
9ab2a3818e6ccefbc6ffdd48df7ecaec25e32e41	CC-BY-NC-ND-1.0	8729	12.7	15.8	14.4	16.4	58.6	13.8	51.0	171
966c97357e3b529e9c8bb8166fbb871c5bc31211	CC-BY-NC-ND-2.0	10074	13.0	16.1	14.7	17.0	59.7	14.3	48.8	206
c659a0e3a5ee8eba94aec903abdef85af353f11f	CC-BY-NC-ND-2.5	10176	12.8	15.9	14.6	16.8	59.2	14.2	49.3	206
ad4d3e6d1fb6f89bbd28a44e263a89430b575dfa	CC-BY-NC-ND-3.0	14356	16.3	19.7	14.1	20.5	66.8	16.2	39.7	361
68960bdf512ff5219909f932b8a81fdb255b4642	CC-BY-NC-ND-4.0	13350	13.3	14.8	15.7	17.2	58.4	14.8	39.4	338
39b2ef67be9e5b4e743e5269a31ad1691515eede	CC-BY-NC-SA-1.0	10228	13.3	16.3	14.2	17.0	59.7	14.2	48.4	211
5800ac2d32e35ace035cdcae693423cd9ff5bb6f	CC-BY-NC-SA-2.0	11927	13.3	16.2	14.7	17.1	60.0	14.4	47.0	253
e5f44c2df6b1391d1ddb6efb2db6f90670e4ae67	CC-BY-NC-SA-2.5	12013	13.1	16.0	14.6	16.9	59.6	14.2	47.7	251
a63b7e81e7b9e30df5d253aed1d2991af47992df	CC-BY-NC-SA-3.0	17134	16.4	19.7	14.2	20.6	67.0	16.3	38.8	441
887f9a5da675cf681421eab3ac6d61f82cf34971	CC-BY-NC-SA-4.0	14577	13.1	14.7	15.7	17.1	58.6	14.7	40.1	363
e4851120f7e75e55b82a2c007ed98ffc962f5fa9	CC-BY-ND-1.0	8280	12.3	15.5	14.3	16.1	57.9	13.6	52.4	158
f1aa9011714f0f91005b4c9eb839bdb2b4760bad	CC-BY-ND-2.0	9228	11.9	14.9	14.5	15.8	56.9	13.5	52.7	175
5f665a8d7ac1b8fbf6b9af6fa5d53cecb05a1bd3	CC-BY-ND-2.5	9330	11.8	14.7	14.4	15.6	56.5	13.4	53.2	175
3fb39a1e46419e83c99e4c9b6731268cbd1591cd	CC-BY-ND-3.0	13591	15.8	19.2	14.1	20.0	65.6	15.9	41.2	329
ac747a640273815cf3a431be0afe4ec5620493e3	CC-BY-ND-4.0	12830	13.0	14.4	15.4	16.9	57.6	14.6	40.7	315
dda55573a1a3a80d294b1bb9e1eeb3a6c722968c	CC-BY-SA-1.0	9779	13.1	16.1	14.2	16.8	59.1	14.0	49.5	197
9cceb80d865e52462983a441904ef037cf3a4576	CC-BY-SA-2.0	11044	12.5	15.3	14.4	16.2	57.9	13.8	50.2	220
662ca9fce7fed61439fcbc27ca0d6db0885718d9	CC-BY-SA-2.5	11130	12.3	15.0	14.4	16.0	57.5	13.6	50.9	218
4a5bb64814336fb26a9e5d36f22896ce4d66f5e0	CC-BY-SA-3.0	17013	16.4	19.8	14.1	20.5	67.2	16.2	38.9	437
8632363dcc2c9fc44f582b14274259b3a35744b2	CC-BY-SA-4.0	14041	12.9	14.4	15.4	16.8	57.8	14.5	41.4	339

It’s good for automated readability metrics that from 3.0 to 4.0 CC-BY-SA is most improved (the relevant clause was a hairball paragraph; CC-BY-NC-SA should have improved less, as it gained the compatibility mechanism) and CC-BY-ND is least improved (it gained express permission for private adaptations).

Next

I leave a list of recommendations (many already mingled in or implied by above) to a future post. But really, just use CC0.