Generally speaking, it is only the public institutions which can research art the most thoroughly as they have a variety of different techniques available to them which can be applied across their extensive collections. Many have access to universities and scientific outlets which enable them to cover every possible avenue of investigation. This has been particularly the case in recent years where scientific infrared techniques have been used to discover all manner of new pieces of information about some of the great works from the past. One example of this would be for The Madonna with the Iris, which we now know received alterations several times over an extended period of time. For example, touches of oil paint have been discovered over the top of a previous use of varnish, which is clearly not something that would have been planned intentionally from the outset of this piece's lifecycle. The painting has therefore grown organically, and this has raised the question around how many artists were involved in its completion.
Upon acquiring the piece in the 1940s, the National Gallery was already unsure about the painting's attribution. Some believed that the abundance of signature elements related to Durer, almost too many, might suggest that actually someone else was trying to pass their own copy off as his. We do know that several versions of this composition can be traced back to this period in the early 16th century and so question marks continue. Initial research concluded that any amendments would have been completed at around the same time, due to the materials used, and so if this was to have been a copy of some sort, it would have occurred at around the artist's own lifetime. The overall conclusion, or most likely explanation, is that this piece derived from the artist's workshop and that a number of different colleagues of his may have been involved, taking inspiration from some of the artworks displayed within the studio at the time. His customary signature would have been added in order to pass this off as his own, perhaps after his death.
It is pleasing to see an institution which is open to the idea of disproving an attribution of an artwork within their own collection, even though its value would instantly drop significantly as a result. It is extremely difficult to draw any strong conclusions about something produced so many centuries ago, and the best one can hope for is an informed conclusion where the acknowledgement of doubt is accompanied alongside the findings. There is nothing to say that Durer himself did not work on parts of this painting himself, and we can see from the initial drawings on canvas that perhaps he helped to shape the work before handing it over to others who would be directed in how to implement the layers of oil. Initial work was carried out in underdrawing, before than being worked over several times.