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18 January 2023| doi: 10.5281/zenodo.7648921

Content moderation on digital platforms: A more intensive horizontal effect of freedom of expression?

To what extent can and should digital platforms be bound by freedom of expression even though they are private companies? This question is central to the issue of content moderation. One solution could be a more intensive horizontal effect of fundamental rights. This article summarises a doctoral thesis on this particular question from a constitutional law perspective.

How social media shape online speech

Digital communication platforms are not neutral actors. From Google blocking ads in Russia and removing YouTube videos that trivialise war, to Twitter refusing to recommend tweets that link to Russian state media, to TikTok suspending all video uploads from the country in response to Russia’s fake news law – all major social media platforms have taken ad hoc measures in recent months that go beyond or contradict their previous policies. For these tech companies, which have become de facto stewards of online information, it is essential to react quickly to global events and change their rules if necessary. But how far does their freedom to decide on the expressions of millions of users extend? When and how should platforms be bound to freedom of expression?

The debate about the increasing privatisation of the public sphere is not new. Examples include the discussions around the privatisation of public space in big cities or of public services. On the internet, however, it is a structural issue. Everything that makes up the internet – from the underlying infrastructure to the visible content on the front end – is offered by private companies while large parts of social life are shifting there. My thesis, which this blog post draws from, focuses on the question of protecting lawful but unwanted speech on the internet, i.e. speech that social networks declare inappropriate and remove. Are social networks indirectly bound by Art. 5 of the German Basic Law if they play a structurally significant role in the free formation of individual and public opinion? How far does the protection of Art. 5 GG extends into private legal relationships and what standards apply to ordinary courts when they review the terms of use of digital platforms?

The doctrine of indirect horizontal effect

To answer this question, my original thesis is dedicated to the doctrine of indirect horizontal effect in constitutional law and deals with its development in the digital society. It examines whethe, according to this doctrine, private actors in the digital sphere could be bound to fundamental rights. In particular, it follows from the recent case law of the Federal Constitutional Court (FCC) that – under certain conditions– private parties can increasingly be bound by freedom of expression in the same way as the state. Since the Lüth ruling of the FCC in 1958, the doctrine of indirect horizontal effect has been applied. According to this doctrine, fundamental rights “radiate” into private legal relationships via certain legal provisions. Courts are required to comply with constitutional standards when interpreting private law.

In the thesis, the analysis of the doctrine of indirect horizontal effect and its application by the FCC has shown that it is a concept that – beyond the principle per se – is not rigid, but can be further developed. It has a high degree of adaptability and is suitable for precisely those private law situations that are highly sensitive to fundamental rights. In the literature, however, the doctrine is accused of being too diffuse and not definite enough in its application by the FCC. In this way, however, the doctrine also retains an openness to new constellations – such as we are now observing on social media platforms. Since the Fraport judgement in 2011 (BVerfGE 128, 226), the FCC has significantly increased the protection of communication freedoms in a private context, with its decisions: Nibelungen (BVerfG, Beschl. v. 18.07.2015 – 1 BvQ 25/15), Stadionverbot (BVerfG, Beschl. v. 11.04.2018 – 1 BvR 3080/09) and III. Weg (BVerfG, Beschl. v. 22.05.2019 – 1 BvQ 42/19)*. Indirect horizontal effect can mean a higher intensity in terms of the binding of private parties to fundamental rights. The intensity of this binding can vary, it does not have to be “state-like or not at all”. Depending on the degree to which these criteria are met, there may be a more intensive horizontal effect between private parties, for example in the context of content moderation. The premise is that private actors provide a place of general communication and social life. Furthermore, it depends on the degree of their dominant position on the market, their orientation and the degree of dependence of the users on that very platform.

Transatlantic perspective

The thesis’ second step is a comparison with the US doctrine of state action. Due to the strict separation between the state and private parties in the US and the non-adaptation of the constitutional doctrine to digital constellations, the differences outweigh the functional equivalence. On the one hand, the Supreme Court has so far refrained from adapting the state action doctrine to circumstances in digital environments. On the other hand, in 2017, the Supreme Court described social networks as a “modern public square” (Packingham v North Carolina). At the same time, this attitude is consistent: both with the concept of the marketplace of ideas and with the primacy of freedom of contract.** It leads to social networks being able to control their users’ expressions of opinion via their terms of use – more than the state would ever be allowed to do in the USA.***

Digital public sphere(s)

The question remains whether and, if so, how the concept of the public must be adapted to the digital context. The recourse of jurisprudence to the concept of “public” is of course not new. This requires a significant transfer between communication science and jurisprudence. This interdisciplinary feedback is also subject to digital change. However, it has not yet been conclusively answered what kind of public(s) is (are) created on the internet and especially through social networks with user-generated content. 

Two factors must be considered here. Firstly, the user is not only a recipient but also an active participant in the information cycle. Secondly, the purpose of use is not exclusively to receive but also to interact socially with others.

Hence, functionalities that are arbitrarily accessible to every user and allow interaction with indefinitely large groups of users correspond most closely to a public forum. But it is precisely there that the importance of platforms as mediators becomes most apparent. One consequence of this is that the curation of digital public spheres must also be protected by fundamental rights. The setting of communication rules is an expression of an orientation – as the FCC indicated in its III. Weg decision. This protection of orientation must be considered.

A more intensive horizontal effect

In summary, the elaborated concept of more intensive horizontal effect serves to give greater weight to certain criteria in the balancing process and, in part, to establish presumption rules that may have to be invalidated. It represents an umbrella concept for the combination of criteria from the case law of the FCC and the findings from research.

As a result, however, the doctrine of horizontal effect remains indirect, because the review of the content of general terms and conditions remains a matter for the courts and there is still no absolute rule of precedence for freedom of expression. In this sense, social networks are allowed to foster their self-defined communication climate in the spaces they provide, but are held accountable to realise freedom of expression.

* These decisions came after the Fraport ruling and have marked a turn in the FCC’s jurisprudence. 
** See also Rodney A. Smolla (2019) The Meaning of the “Marketplace of Ideas” in First Amendment Law, Communication Law and Policy, 24:4, 437-475, DOI: 10.1080/10811680.2019.1660552
*** See also Heldt, A. P. (2019). Merging the Social and the Public: How Social Media Platforms Could Be a New Public Forum. Mitchell Hamline L. Rev., 46, 997.


Castells, M. (2008). The new public sphere: Global civil society, communication networks, and global governance. The aNNalS of the american academy of Political and Social Science, 616(1), 78-93.

Gillespie, T. (2018). Custodians of the Internet: Platforms, content moderation, and the hidden decisions that shape social media. Yale University Press.

Klonick, K. (2017). The new governors: The people, rules, and processes governing online speech. Harv. L. Rev., 131, 1598.

Roberts, S. T. (2019). Behind the screen. Yale University Press.

This post represents the view of the author and does not necessarily represent the view of the institute itself. For more information about the topics of these articles and associated research projects, please contact

Amélie Heldt

Former Associated Researcher: Platform Governance

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