The following chart plots the month returns of the Nikkei 225 Index in January for the period 1980-2017.
In the 40 years from 1950 to 1989, the Nikkei 225 Index only fell in January in 6 years. After that, as can be seen in the above chart, the record became quite a bit more patchy. For example, in the 10 years since 2008, the Index has fallen over 8% in January four times.
Further analysis of the Nikkei 225 Index in January over different time periods can be seen in the following table.
In the 68 years from 1950 to 2017 the Index had an average month return in January of 2.5%, and saw positive returns in 69% of years. But since year 2000 this has dramatically changed (as was also the case of the US and UK markets). Since 2000, the Index has had an average return in January of -1.6%, the worst average return of any month in this period.
The following charts plots the cumulative returns for the 12 respective months since 1980 (for more explanation of this chart see here).
The cumulative portfolio for January has been highlighted in the above chart.
The cumulative performance of January peaked in 2001, at which point it was the best performing month in the year. Since 2001, the cumulative performance has dramatically under-performed that of other months.