Geography Reference
In-Depth Information
The evidence reported by JTH shows indeed that citations are highly localized. Citing
patents are up to two times more likely than the control patents to come from the same
state, and up to six times more likely to come from the same metropolitan area.
Agrawal et al. (2006) propose a variation on the original JTH experiment that sheds
some light on the nature and durability in time of social ties. In particular, they focus
on the citations received by patents signed by 'movers', the latter being inventors who
appear to have changed location over time (that is, they have signed at least two patents,
reporting at least once a dif erent address). To identify such patents, each citing-cited
(control-cited) patent pair is 'unbundled' in order to obtain as many observations as the
number of co-inventors listed on the originating patent; that is, from citing(control)-
cited pairs, one obtains several so-called citing(control)-cited-inventor 'triples'. This
allows identifying the relevant movers and their patents, whose 'triples' are then retained
for running the co-location test. The latter is performed considering both the movers'
'current' location (the city where the mover resided at the time of the patent application)
and the 'prior' ones (the city/cities where the mover used to reside at an earlier time, as
witnessed by his or her earlier patent applications).
Agrawal et al. i nd that movers' patents are more heavily cited than expected not only
by inventors resident in the movers' current locations, but also by those who reside in the
prior ones. Among the latter, inventors with a past working spell in the same companies
as the movers appear to be the majority. These results suggest that movers keep in touch
with their former colleagues, or at least that they leave a lasting knowledge legacy behind
them.
Interpretation and the role of social networks
The mobility of R&D scientists and engineers within a localized labour market and
the existence of localized markets for technologies have both been reported by various
authors as potential explanations of JTH results
4
.
As for labour mobility, Almeida and Kogut (1999) have replicated the JTH exercise
for each US state. They i nd evidence of localized knowledge l ows only in those few
regions (most notably, the Silicon Valley) where the intra-regional mobility of inventors
across companies is high.
Markets for technologies may also explain the JTH results to the extent that co-
location is encouraged by technology users' need to consult frequently with their
suppliers. Research contracts signed by the same independent inventor with dif erent
companies may produce patents that appear to be unrelated in terms of ownership, but
very close in terms of technological contents and geographical distance (Mowery and
Ziedonis, 2001).
The above-mentioned studies suggest that patents linked by a citation may also be
personally or socially linked. A personal tie occurs whenever the same inventor is respon-
sible for two patents from two dif erent companies, either because he or she moved from
one to another, or because he or she is an independent inventor who sold ideas to both.
A social tie exists whenever two inventors
i
and
j
working for two dif erent companies
have a common professional acquaintance, in this case a fellow inventor who has worked
jointly on a patent both with
i
and
j
(he/she may be either an employee who has moved
across the two companies, or an independent professional who has consulted or done
research for both of them). Other social ties between two patents may involve a longer
