In addition to the legal uncertainty, licenses also create the risk of imposing burdensome attribution requirements. In the science context in particular, projects often rely on data gathered from a variety of different sources. Depending on the licenses used, it is possible that would require attributing each individual or institution that contributed any piece of data to the project. This is a problem we call attribution stacking.
This raises yet another potential problem with attribution. Attribution obligations written into a license are, by their nature, inflexible. No lawyer can anticipate every situation in which the attribution requirements would be triggered and account for all of the circumstances in which they will be applied. This can create some absurd situations where, for example, a user or aggregator of data may technically be required to attribute 1000 different data providers, all in the idiosyncratic manner that the rights holder has dictated. Conceivably, the user could do all this and still not satisfy people’s expectations for receiving credit or accepted standards of citation.
The next legal mechanism for requiring attribution is contract law. Contracts can have different names and take a lot of different forms, but they are often called data use agreements or data access policies.
Unlike a license, a contract does not necessarily require an underlying intellectual property right. Technically, it requires a few legal formalities, including an offer and acceptance. In practice, sometimes that manifests in an online agreement, where the user has to click to accept the terms to access to data. Other times the user is presumed to have accepted the terms by continuing to use the site. If you read those terms, they may require attribution.
Like licenses, contracts suffer from a number of potential downsides. For one, they likely impose confusing obligations on users who get data from a variety of sources, all subject to different user agreements. This problem is even more pronounced with contracts because at least public licenses are somewhat standardized. User agreements are not, which means each data source likely has a different user agreement, filled with legalese imposing attribution and other obligations on users. The consequence is that some data sources may not be used simply because users cannot understand the terms.
Another limit to contract law is that it only binds the parties to the agreement. That may sound obvious, but this is not the case with licenses. If someone obtains licensed data and shares them, the person who obtains them it from that second user is still bound by the conditions of the license. If the data were shared by contract alone, the person who obtained the data from the second user would not be bound by the terms of the contract because they were not a party to the original agreement. In this respect, contracts have a more limited reach than licenses.