Geography Reference
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chain of acquaintances, such as when inventors i and j are connected by two other inven-
tors z and w who i rst worked together, and then moved on to work separately with i and
j , and so on.
More indirect social ties may also exist, such as serendipitous encounters between any
two inventors who meet at workshops or through friends and other non-professional
acquaintances. We fail to capture these ties with patent data, which record only formal
collaboration instances.
In principle, social ties may or may not be concentrated in the geographical space:
robust social links between inventors may convey tacit information even when the inven-
tors have just a few chances to meet personally (witness many fruitful academic coopera-
tion experiences), or well after the inventors have last met.
Agrawal et al.'s (2006) extension of the JTH experiment improves our understanding
of what types of social tie really matter for knowledge dif usion. Still, it tells us little of
who knows whom in the inventors' community. We know that patents are dispropor-
tionally concentrated in the hands of relatively few large companies, wherein inventors
may or may not have a chance to meet and exchange knowledge, or get in touch with
each other's ideas via chains of mutual acquaintances; at the same time, these companies
may provide for the codii cation of their inventors' knowledge in order to help its circula-
tion among all technical employees.
Both the original JTH experiment and its extension by Agrawal et al. (2006) make use
of a database with limited information on inventors' social capital. None of the databases
contains information on the inventors' social ties; as a consequence, both geographical
distance and company ai liations end up being two summary proxies for all kinds of ties,
and for physical distance as such.
Having reclassii ed patents according to inventors, we will be able to show the impor-
tance of social ties as distinct from spatial localization. In particular, we will emphasize
the role of a specii c professional community (that of inventors) as a channel of knowl-
edge dif usion, and show that the localization of knowledge l ows is largely explained by
the limited mobility in space of such a community. Thus, we will 'open the black box'
of localized social ties, often evoked as the conduit of knowledge, but seldom told apart
from one another and subject to measurement.
We regard our exercise as the i rst step towards a more comprehensive direct measure-
ment of all social ties between inventors and, more generally, knowledge producers. It is
only by detecting what types of social tie matter (in our case, we focus on collaboration
ties between inventors, recorded by patent documents) that researchers will be able to
explain why spatial distance is often found to limit knowledge dif usion, and possibly
reach strategic or policy prescriptions.
3.
Methodology and data
Social networks: dei nition and methodology
Our methodology exploits patent-recorded information on inventors' names, surnames,
addresses, and company ai liation (Breschi and Lissoni, 2004 and 2009). The follow-
ing hypothetical example illustrates the main idea (see Figure 16.1). Let's consider i ve
patents (1 to 5) and four assignees (a, b, g, d). Assignee a owns two patents (1 and 2),
while assignees b, g and d one each. Patents have been produced by 13 distinct inventors
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