1. In December 2016, I wrote a short Amazon book on the economics of revealed preference.  I wrote this book after teaching "Econ 101" at USC.  While I have taught Econ 101 on and off for 25 years (starting back at Columbia University), I have now concluded that the right way to teach this class is to cast the economist as "a detective".   We observe clues about a person's "type" based on the choices we observe her make when she is confronted with different choices sets (that vary due to her income changing and the relative prices she faces shifting).  Under the assumption that a person's tastes do not change much over time, we can begin to pin down a specific person's tastes.

    An example of my book's logic.   Sally is offered a meat pizza at a price of $15 and she doesn't buy it.  The next day, she is offered a meat pizza for a price of $2 and she doesn't buy it.  The next day, she is offered a meat pizza for 1 penny and she doesn't buy it.   We begin to deduce that Sally is a vegetarian.    While we don't know Sally, we observe Sally's market choices and we learn about her.  This is the the theme of my book.  We take revealed preference seriously and thus are able to "reverse engineer" what must be the preferences of the consumes whose choices we observe.  This is the right way to teach econ 101.  Data combined with theory yields insights about diverse peoples' types.

    Note that this is very different standpoint then what is usually taught in intermediate micro where I tell you Sally's preferences and budget constraint and you grind out her optimal consumption bundle using simple calculus or geometry. I argue in my 2016 book that we should solely study the "inverse problem" focused on partially identified models.  What do I learn about you based on the subset of choices I observe you make?


  2. Now that 2017 is wrapping up it might interest some people to hear why I choose to work on some research questions.  For each paper I published in 2017, I offer a few "big picture" comments to explain what questions motivated the research.

    2017 Articles

    1.  Jerch, Rhiannon & Kahn, Matthew E. & Li, Shanjun, 2017. "The efficiency of local government: The role of privatization and public sector unions,"Journal of Public Economics, Elsevier, vol. 154(C), pages 95-121.
    2.  Delmas, Magali A. & Kahn, Matthew E. & Locke, Stephen L., 2017. "The private and social consequences of purchasing an electric vehicle and solar panels: Evidence from California," Research in Economics, Elsevier, vol. 71(2), pages 225-235.
    3.  Dora L. Costa & Matthew E. Kahn, 2017. "Death and the Media: Infectious Disease Reporting During the Health Transition," Economica, London School of Economics and Political Science, vol. 84(335), pages 393-416, July.
    4.  Bunten, Devin & Kahn, Matthew E., 2017. "Optimal real estate capital durability and localized climate change disaster risk," Journal of Housing Economics, Elsevier, vol. 36(C), pages 1-7.
    5.  Matthew E. Kahn, 2017. "Will Climate Change Cause Enormous Social Costs for Poor Asian Cities?," Asian Development Review, MIT Press, vol. 34(2), pages 229-248, September.
    6.  Sun, Cong & Kahn, Matthew E. & Zheng, Siqi, 2017. "Self-protection investment exacerbates air pollution exposure inequality in urban China,"Ecological Economics, Elsevier, vol. 131(C), pages 468-474.
    7.  Siqi Zheng & Matthew E. Kahn, 2017. "A New Era of Pollution Progress in Urban China?," Journal of Economic Perspectives, American Economic Association, vol. 31(1), pages 71-92, Winter.

    1.  It is difficult to rank the productivity of local governments in providing services at a point in time or to compare the same city's productivity over time.  We argue that the average cost of moving a bus a mile offers an "apples to apples" metric for ranking local governments.  We study when governments outsource to private entities and how such outsourcing affects the average cost of service delivery.  We explicitly model the endogenous privatization choice.

    2.  Solar panels and EVs are complements.  It is no accident that Tesla and Solar City have merged into one company.  We use existing data to study the joint spatial distribution of the purchases of these "green products".   We discuss how innovations in financing these durables affects their annual "affordability".

    3.   How does the NY Times decide what to report?  Major cities report their death rates from infectious diseases several times a year. Such dynamics in this "objective reality" provide a benchmark for studying whether the media reports "good news".  During times of slow linear progress in death rates, the media doesn't cover the story.

    4.   The popular media routinely publishes pieces arguing that coastal real estate in cities such as Miami is doomed because of sea level rise caused by climate change.  If forward looking developers and real estate owners are aware of these emerging risks, then this alters development patterns and capital maintenance patterns.  The net effect of such supply side decisions is a more elastic supply of housing and less ex-post damage caused by the sea level rise that ultimately occurs. The New York Times and other (i.e Joe Romm) implicitly assume that the capital stock in at risk places is infinitely lived. While this capital is durable, it has a finite (and endogenous) life.  One way to adapt to climate change is to have a  less durable capital stock such that we retain the option to rebuild on higher ground.  This supply side approach to studying real estate in the presence of emerging spatial risks has not been previously studied.


    5.  This piece reviews my thinking about how cities in LDC Asian nations (think of Thailand) will adapt to climate change. My piece is realistic but optimistic about the role that free markets play in protecting such individuals against emerging risks.

    6.  This piece uses city/day level data on sales of air filters and masks to study how rich and poor people in China adapt to high levels of urban air pollution.  Since masks are cheap, both the rich and poor are more likely to buy them on more polluted days. Since air filters are expensive, only the rich buy them on more polluted days.  Such differential investment in self protection means that the rich are better able to self protect against pollution. This means that pollution exacerbates overall inequality between the rich and poor in urban China.

    7.  This paper presents the "big ideas" of our 2016 Princeton Press book. Our book presents a new twist on the 1990s Environmental Kuznets Curve hypothesis.  We closely link together ideas from urban and environmental economics to explain how China's cities became so polluted and why pollution progress is now starting.  We sketch out an urban political economy for why mayors in some cities are pursuing the "green agenda" while others are not.  




  3. The recent Los Angeles fires have been quite scary.  When I'm scared, I start to run new regressions.  I take daily PM2.5 air pollution data from the EPA and keep the subset of observations for the following states; California, Arizona and Nevada.  I use data from the years 2000 to 2017.   I merge in data on daily wind speed and average temperature.  I create two key variables;

    Fireseason = 1 if the day is in September, October, November, December

    trend  =  the monthly time trend (so there are 17 years and 12 months per year so this variable takes on the values 1 to 12*17)

    inter  =  Fireseason*trend   , this interaction term tests if the monthly time trend differs during the fire season.

    So, for 152 monitoring stations in Arizona, California and Nevada; I am running a regression where the Y variable is the log(PM2.5) on a given day.  I include monitoring station fixed effects and I regress this on a monthly time trend (t),  the fireseason dummy (see above) the time trend during the fireseason months and the climate variables (log wind and log average temperature).



    . areg lmean t inter  fireseason lwind lavgtemp , absorb(idno) cluster(idno)

    Linear regression, absorbing indicators         Number of obs     =    405,388
                                                    F(   5,    151)   =      47.76
                                                    Prob > F          =     0.0000
                                                    R-squared         =     0.2775
                                                    Adj R-squared     =     0.2772
                                                    Root MSE          =     0.6412

                                     (Std. Err. adjusted for 152 clusters in idno)
    ------------------------------------------------------------------------------
                 |               Robust
           lmean |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
    -------------+----------------------------------------------------------------
               t |  -.0018944   .0002363    -8.02   0.000    -.0023612   -.0014276
           inter |   -.001042   .0002175    -4.79   0.000    -.0014717   -.0006123
      fireseason |   .2643525   .0310733     8.51   0.000     .2029578    .3257471
           lwind |  -.1425646   .0743011    -1.92   0.057    -.2893687    .0042395
        lavgtemp |  -.0306948    .072866    -0.42   0.674    -.1746633    .1132738
           _cons |   3.133668   .5095623     6.15   0.000     2.126875     4.14046
    -------------+----------------------------------------------------------------
            idno |   absorbed                                     (152 categories)


    What have we learned?    All else equal, West coast PM2.5 is 26.4% higher during the fire months.  But, for those who are looking for optimism --- note the time trends.  The monthly time trend = -.0018 , so for every 10 months that pass; PM2.5 declines by 1.8% in the non-fire season months. Note that inter has a negative coefficient of -.001.  During the fire season months;  the downward trend in PM2.5 is even more pronounced.    During the fireseason, PM2.5 is trending down at about 3% per year.

    These results are roughly the same when I weight by county population size (so I'm placing more importance on LA county). Note the time trend is even more negative in this case.




    . areg lmean t inter  fireseason lwind lavgtemp  [w=pop2000], absorb(idno) clus
    > ter(idno)
    (analytic weights assumed)
    (analytic weights assumed)
    (sum of wgt is   6.6541e+11)

    Linear regression, absorbing indicators         Number of obs     =    405,388
                                                    F(   5,    151)   =      64.11
                                                    Prob > F          =     0.0000
                                                    R-squared         =     0.3227
                                                    Adj R-squared     =     0.3224
                                                    Root MSE          =     0.5881

                                     (Std. Err. adjusted for 152 clusters in idno)
    ------------------------------------------------------------------------------
                 |               Robust
           lmean |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
    -------------+----------------------------------------------------------------
               t |  -.0024729   .0002347   -10.54   0.000    -.0029366   -.0020092
           inter |  -.0010034   .0002073    -4.84   0.000     -.001413   -.0005938
      fireseason |   .2544869   .0254936     9.98   0.000     .2041168     .304857
           lwind |   -.002755   .0483044    -0.06   0.955    -.0981948    .0926848
        lavgtemp |   .0321614   .1147983     0.28   0.780    -.1946571    .2589798
           _cons |   2.422674   .5416897     4.47   0.000     1.352404    3.492944
    -------------+----------------------------------------------------------------
            idno |   absorbed                                     (152 categories)


    Fires are certainly horrible but the drought prone region is suffering less PM2.5 pollution over time.  These regressions cannot tell you why this is the case.

    Here are the results just for California.


    . areg lmean t inter  fireseason lwind lavgtemp if state==6, absorb(idno) clust
    > er(idno)

    Linear regression, absorbing indicators         Number of obs     =    318,986
                                                    F(   5,    118)   =      53.93
                                                    Prob > F          =     0.0000
                                                    R-squared         =     0.2530
                                                    Adj R-squared     =     0.2527
                                                    Root MSE          =     0.6482

                                     (Std. Err. adjusted for 119 clusters in idno)
    ------------------------------------------------------------------------------
                 |               Robust
           lmean |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
    -------------+----------------------------------------------------------------
               t |  -.0017874   .0002511    -7.12   0.000    -.0022846   -.0012902
           inter |  -.0012424   .0002473    -5.02   0.000    -.0017321   -.0007527
      fireseason |   .2895557   .0306528     9.45   0.000     .2288548    .3502565
           lwind |  -.1454173   .0835908    -1.74   0.085    -.3109498    .0201152
        lavgtemp |     .03904   .0957566     0.41   0.684    -.1505842    .2286641
           _cons |   2.894797   .6436804     4.50   0.000     1.620135    4.169459
    -------------+----------------------------------------------------------------
            idno |   absorbed                                     (119 categories)





  4. The Repec competition continues.  I do not believe that Martin Browning is part of our cohort. 

    1993


    RankAuthorScore
    1John Michael van Reenen 2.22
    2Florencio Lopez-de-Silanes
    SKEMA Business School, Lille, France
    2.51
    3James Alan Robinson 3.44
    4Charles I. Jones 4.33
    5Martin James Browning 6.88
    6Serena Ng 7.31
    7Thomas Piketty 7.6
    8Matthew E. Kahn 7.98
    9Casey Mulligan 11.75
    10Mike Wright
    Business School, University of Nottingham, Nottingham, United Kingdom


    Repec has informed me that my rankings "peers" are:


  5. Under the pending Trump Tax Plan, Universities whose endowments are above $500,000 per student will face an endowment income tax of 1% a year.  A $7 billion dollar school would pay roughly $10 million dollars in cash (that's a lot of Assistant professors slots).  Holding a university's endowment fixed, if this school increases its student population by 20%, it will be less likely to face this tax threshold.  From the link above, I see that Duke, UPENN, Columbia and University of Chicago are all hovering around the threshold.  I predict that they will be admitting more students in the near future.

    My son is a junior in high school so this incentive effect is of interest to me.

    UPDATE:  This post has been updated to correct my mistake.  The tax is on endowment income (not wealth).  I thank USC Price Professor Nick Duquette for updating me. 
  6. Given California's high taxes on those who are well paid, such individuals keep 45% of each dollar they earn.  If such a person lived in Texas, he might keep 60% of each dollar earned.   President Trump's new tax proposal will further raise the tax price of living in California.  If state and local taxes are no longer deductible from federal income, then the tax price will rise sharply.     Who will bear the incidence of this tax?  Will even progressive voters in California demand a smaller government?  Will UCLA and the other public schools suffer?  Will the rich now start to move to lower tax areas or does CA's unique amenities keep people here despite the high tax?
  7. At USC Economics,  I (in my role as department Chair) have a busy schedule. We are hiring new faculty, creating new curriculum and engaging in fundraising.  Each day brings new challenges and new opportunities.   On  Tuesday, I will be interacting with our talented majors outside of the classroom in two different events;

    Noon:  Lunch with Undergraduates

    6pm  The Economics of Bitcoins

    A department chair is a type of cheerleader.  I try to convey an optimism and enthusiasm for what we are trying to achieve. I try to discourage free riding and I try to celebrate effort and public goods provision. 

    I keep thinking back to my undergraduate education and I ask myself; what events would I have learned from if I had had these opportunities 30 years ago?  Based on my memories of my past, I keep trying to schedule meaningful (and fun) events.


  8. This new NBER paper looks quite interesting.  Suppose that I am an inventor and I create a blood test that correctly detects your probability of having cancer .5 years from now.  For my test to accurately predict your future risk probability, a trained nurse must administer the blood sampling.   As I debut my product, I will make sure that the qualified nurse does the test.    The Scaling up problem is that if my test starts to sell by the millions, its accuracy will decline because there aren't enough qualified nurses to administer my test.

    So, this isn't a case of constant returns to scale.  As my production rises, the quality of my product declines and the RCT economists would say that my pilot study's average treatment effect over-states the average treatment effect as I scale up the size of my market.

    But is this true?   In a rational expectations model, If I can pre-commit that my product will be "big" (both in scale and with regards to its benefits if administered by a well trained nurse), then young nurses will train to be experts in my technology and will obtain the human capital necessary to operate my product. In this case, diminishing returns to my product may not kick in.  I cannot credible signal my future success then a co-ordination failure will occur and the empirical researcher will observe that the average treatment effect declines with my scale of production (because I can't find qualified nurses to administer it).

    Alternatively, suppose that I only roll out my product in California.  High quality nurses may start to move to California because they can work with my technology and deliver results. In this case, my product's average treatment effect will not decline with the scale of my sales.

    So, when there is a complementarity between a treatment (such as my blood test) and human skills --- the key issue for scale up is "rational expectations and market size"  or "migration and general equilibrium". 

    Scarce inputs will not remain scarce for long if we receive a "heads up" of rising demand (i.e high nurse earnings for skilled nurses) or high demand in given spatial location (i.e nurse salaries for high skilled nurses in California).  As usual, the shape of the supply curve in the long run versus the short run plays the key role here.   RCT results can be scaled up if the complementary input is elastically supplied.  If it isn't then you must ask, why isn't it?  What is the barrier to entry?

    Note that this is a blog post. I am ignoring "essential heterogeneity" of those at risk to be treated. Instead, I am focusing on the endogenous determination of the treatment's quality (not the demander's response to this treatment).  #supplymatters
  9. Sherwin Rosen was one of the greatest University of Chicago economists.  While he did not win a Nobel Prize (he died at age 62 during the year when he was the President of the American Economic Association), his student Richard Thaler won the Nobel Prize and his student Kevin Murphy has won multiple major economics honors.   I was not his best student but he continues to teach me new lessons about economics.   I just read his 1997 paper on Austrian Economics.  I now see that my Climatopolis work is a type of Austrian Economics. 

    My 2010 book (see the short version here) argues that the combination of rising urbanization, human capital and innovation together will allow us to adapt to climate change.  Cities compete for the skilled and those cities that successfully adapt to the challenge of climate change will gain in human capital.  Home prices (and thus income effects) will fall in areas that fail to adapt.  This competition and the potential for migration creates a more overall resilient economy. While I cannot tell you today which cities will win this competition, I am very confident in this "Austrian" vision.

    At the same time that I continue this work, there are plenty of NBER environmental economics researchers estimating reduced form single equation models of the general form:

    economic outcome =  a + b*climate conditions +  U

    for example, the outcome variable might be mortality, or worker productivity and the key explanatory variable might be annual days of the year that the temperature is over 100 degrees.

    Researchers seek out "credible research" designs to estimate "b".  This slope represents the current marginal effect of climate on an economic outcome.  This research ignores cross-elasticities.  If the climate is bad in Kansas but great in Oklahoma and expected to remain so,  the negative shock to Kansas will actually create a boom in Oklahoma.  This is a migration (zero sum game) effect.
    Yes, a migration cost must be paid but this is a 2nd order effect.

    Given my read of Sherwin Rosen's paper, I now see that Austrian Economics focuses on the evolution of the economic system.  Entrepreneurs intuit that there is emerging demand for this product (think of Uber) and begin the experimentation to develop it.  Some succeed and some fail.  The system evolves to economize on scarce resources (signaled by prices) that may becoming increasingly scarce.

    What can NBER's empiricists actually do here to satisfy an Austrian economist's vision?  One empirical agenda is to study emerging venture capital fund investments. Another would be to study patenting behavior in key sectors affected by climate change.   Investment under Knightian uncertainty is an under-researched topic. In this case, firms know that they do not know the future for certain but they foresee certain emerging trends such as increased drought conditions in the American West. Rather than being passive victims, some firms see this as an opportunity and this starts the endogenous R&D progress.

    Note that the empirical researcher who assumes she is studying a stationary process will begin to observe that the "b" coefficient defined above converges to zero over time (perhaps not in a linear fashion) as lumpy new innovations are developed and brought to market.  This point is one of our key points in this 2017 paper.









  10. Here are two photos I took of our new neighbor.




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