This study attempted to represent psychological development curves separately for males and females using polynomial equations. We utilized standardized data from the 2020 edition of the Kyoto Scale of Psychological Development (KSPD) to specify these equations. The participants included 1281 males and 1355 females ranging in age from 0 to 20 years. This age range was chosen because, after the age of 20, there is little to no increase in KSPD scores. Using the least squares method, polynomial equations from the first to the sixth degree were calculated, with age (in days) as the x-axis and scores as the y-axis. As a result, for both males and females, no differences were observed in the coefficient of determination for second-degree or higher equations, with the highest average agreement rate found in third-degree equations (males 96.0%, females 95.7%). The cubic equation for males was y= 4.272･10－1x－4.592･10－5x2+ 1.586･10－9x3+1.293 (R2=.986), and the cubic equation for females was y= 4.284･10－1x－4.623･10－5x2+ 1.437･10－9x3+3.932 (R2=.985). We determined that the third-degree equation provided the best fit for both males and females and calculated the maximal value. The maximal value was calculated because it is estimated to correspond to the peak of development. The maximal value for males was x = 7818.5, y = 1292.3, while for females, it was x = 6770.9, y = 1231.2. In other words, males were estimated to reach their developmental peak at 7818.5 days (256.9 months: 21.4 years) with a score of 1292.3, while females were estimated to reach their peak at 6770.9 days (222.5months: 18.5 years) with a score of 1231.2. These results may suggest slight differences in the developmental timing between males and females.